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The fundamental limits of atomic fountains as operational clocks are considered. Four rubidium fountains in operation at the US Naval Observatory for over 5.5 years have demonstrated unprecedented long-term stability for continuously running clocks (Peil et al 2014 Metrologia 51 263–9, Peil et al 2016 J. Phys.: Conf. Ser. 723 012004). With only these rubidium fountains, a post-processed timescale can be created that demonstrates superior long-term performance to any individual clock by compensating for occasional frequency steps. By comparing to the world's primary standards we demonstrate instability of this rubidium fountain timescale reaching the mid 10−17's and zero drift at the level of 1.3 × 10−19 d−1. We discuss fundamental limits due to common mode behaviour or individual fountain performance that cannot be corrected.

With the continuous development of AI intelligent technology, ornamental water resource design has been applied in various fields, and the interactive application of new music fountain and computer is also more extensive, which makes more control circuits need to be completed by computer, and it is developing towards intelligence, dispersion, synthesis and diversity. Artificial fountain music usually has two system composition, one is the control program system, main control the style and flow of water, the other is music control system, mainly through computer programming code, and according to different music, programming, so as to achieve shape synchronous music and water combined to send out a different style of artistic effect. Musical fountain landscape entertainment application scene extension is quite rich, the fountain can be designed out of different patterns, through the program programming according to a certain law to control the pattern of water, make it colorful, different shapes. Many squares include large entertainment venues, plus music fountain elements, the overall beauty can be improved, the design of an energy-saving pattern fountain controller in line with the mainstream of the times.

The fabrication of molecular films by coating a conductive polymer solution on a substrate is an active topic in molecular electronics research. Since the inhomogeneity of polymer thin films modulates the chemical states and electrical properties of these films, the construction of integrated polymer devices requires methods to fabricate controllable, homogeneous molecular films as small patterns. Herein, we report the reproducible fabrication of sub-micrometer thickness self-doped polyaniline lines with high homogeneity by fountain-pen lithography (FPL). Compared with commonly used drop-casting and spin-coating methods, the fabrication of line patterns by FPL allows good control of line size, chemical/micro structures, and electronic properties, as demonstrated here by microscope Raman spectroscopy and conductance measurements. Supplying an identical amount of solution from a nanopipette and rapid solvent evaporation are likely crucial for fabricating homogeneous line patterns. This method is promising for the development of molecular electronic devices and the characterization of polymeric materials.

We theoretically predicted and experimentally demonstrated the sinusoidal shifts of the 87Rb atom Raman transition frequencies as the polarization vectors of the Raman lasers rotate. We also presented the vanish of the differential light shifts for the magnetic sensitive Raman transitions at some elliptically polarized configurations while no vanish for the magnetic insensitive Raman transition. These results are helpful for the evaluation and elimination of the light shifts induced systemic errors in the atomic magnetometer, atomic interferometer, atomic clock and quantum computing.

If the end of a long chain, which is contained in an elevated beaker, is dropped over the edge of the beaker and falls, it is observed that as the speed of the chain increases the chain rises to form a loop well above the top of the beaker. The name 'chain fountain' has been applied to this phenomenon. In this study the dependence of the fountain loop height on the height the chain falls to the ground is investigated by the measurement of the heights over a range greater than those previously reported. The data are compared with earlier work and model predictions. It is found that the linear dependence of the fountain height on the fall height as predicted by the considered model is confirmed for heights below 3.5 m. However, it is observed that when the range of fall heights is extended well beyond this the dependence is strongly non-linear.

The cesium fountain primary frequency standard NIM5 has been developed at the National Institute of Metrology in China. The NIM5 loads atoms in an optical molasses from the background Cs vapor directly. Atoms are then cooled to a temperature of about 2 μK and launched to a height of 81 cm. The fringes of the Ramsey pattern have a width of 0.98 Hz. The NIM5 operates for more than 300 d a year, operating nearly continuously for 15 d at a time. By stabilizing the 9.19 GHz microwave frequency to the center of the central Ramsey fringe, a typical fractional frequency instability of 3 × 10−13 (τ/s)−1/2 is obtained when running at high atom density, and a combined uncertainty, including Type A and B uncertainties, is typically 1.6 × 10−15. Comparisons of data between NIM5 and 5 other fountain clocks were carried out in May 2013 via two-way satellite time and frequency transfer (TWSTFT), and the results show good agreement within the uncertainties. Six groups of NIM5 data from January to June 2014 have been published in Circular T 319 and 320.

We report on the first comparison of distant caesium fountain primary frequency standards (PFSs) via an optical fiber link. The 1415 km long optical link connects two PFSs at LNE-SYRTE (Laboratoire National de m\'{e}trologie et d'Essais - SYst\`{e}me de R\'{e}f\'{e}rences Temps-Espace) in Paris (France) with two at PTB (Physikalisch-Technische Bundesanstalt) in Braunschweig (Germany). For a long time, these PFSs have been major contributors to accuracy of the International Atomic Time (TAI), with stated accuracies of around $3\times 10^{-16}$. They have also been the references for a number of absolute measurements of clock transition frequencies in various optical frequency standards in view of a future redefinition of the second. The phase coherent optical frequency transfer via a stabilized telecom fiber link enables far better resolution than any other means of frequency transfer based on satellite links. The agreement for each pair of distant fountains compared is well within the combined uncertainty of a few 10$^{-16}$ for all the comparisons, which fully supports the stated PFSs' uncertainties. The comparison also includes a rubidium fountain frequency standard participating in the steering of TAI and enables a new absolute determination of the $^{87}$Rb ground state hyperfine transition frequency with an uncertainty of $3.1\times 10^{-16}$. This paper is dedicated to the memory of Andr\'{e} Clairon, who passed away on the 24$^{th}$ of December 2015, for his pioneering and long-lasting efforts in atomic fountains. He also pioneered optical links from as early as 1997.

Earthquakes represent huge risks for cultural heritage. Although there is a large literature about prevention on historical buildings, the seismic assessment on collections and exhibition designs is usually underestimated. "RESIMUS" long running project involves different knowledge and fields; its goal is to prevent seismic damages to museum collections and artefacts, combining both museographical solutions and anti-seismic methods. This contribution focuses on the "Fontana di Giunone", designed in 1555 by Bartolomeo Ammannati and currently kept at the Museo Nazionale del Bargello. The museographical and historical researches have been combined with the seismic performance of the reconstructed Fountain. In this paper the first results provided by the seismic analysis of this artefact are shown. A 3D structural model has been set, based on the three dimensional digital model provided by an detailed laser scanner survey. An elastic dynamic analysis has been performed, by representing the seismic input through an ensemble of seven ground motions expressing the seismic hazard of the area. The analysis has provided the seismic response of the sculptures, evidencing its sensitivity to the dynamic properties of the considered ground motions.

The uncertainty evaluation of CSF2, the second caesium fountain primary frequency standard at PTB, is presented. The fountain uses optical molasses to cool atoms down to 0.6 µK. The atoms are launched vertically in a moving optical molasses, and state selected in the |F = 3, mF = 0 hyperfine ground state. During their ballistic flight, the atoms interact twice with a microwave field, thus completing the Ramsey interaction. With a launch height of 36.5 cm above the cavity centre, the central Ramsey fringe has a width of 0.9 Hz. About 3 × 104 atoms, 30% of the initial number in the |F = 3, mF = 0 state, are detected after their second interaction with the microwave field. Stabilizing the microwave frequency to the centre of the central Ramsey fringe, a typical relative frequency instability of 2.5 × 10−13(τ/s)−1/2 is obtained. The CSF2 systematic uncertainty for realizing the SI second is estimated as 0.80 × 10−15. First comparisons with the fountain CSF1 at the Physikalisch-Technische Bundesanstalt and other fountain frequency standards worldwide demonstrate agreement within the stated uncertainties.

There are currently nine caesium fountain primary frequency standards regularly reporting calibrations of International Atomic Time to the Bureau International des Poids et Measures (BIPM). An investigation has been carried out using data from the BIPM publication Circular T to evaluate the frequency differences among these standards and to determine whether these offsets are consistent with the stated uncertainties. The fractional frequency uncertainties of some Cs fountains are now in the range of 4 × 10−16 to 5 × 10−16. The results of this investigation show that the standards agree well with each other. An overall estimate of the caesium frequency is made using the weighted mean of all the fountains.

TA(NIM) is being updated now. The new TA(NIM) uses an active hydrogen maser as a reference clock, and the frequency of TA(NIM) is steered by an algorithm to keep it the same as the NIM4 fountain clock, which is NIM's primary frequency standard. This paper introduces the generation of new TA(NIM), including the equations to predict maser frequency and the steering algorithm. The time difference and the frequency difference between the new TA(NIM) and UTC are also reported.

I describe an unusual fountain that produces a jet using the siphon principle. Fountains have always attracted and fascinated people from the time of the ancient Greeks to the present day, and during this period there have been several ways in which fountains have operated based on different laws of physics. The fountain that I describe below is a siphon fountain and the principle on which it works dates back to the end of the 18th century and may be attributable to the French nobleman the Marquis of Mannoury D'Ectot (1777–1822) (Comi 1995 Giornale di Fisica 36 (1)).

We have employed an intuitive model and 3D Monte Carlo simulations to study the effect of ballistic expansion of the atomic cloud on the collision energies and the resulting frequency shift in a fountain clock. In particular, we show that the energies relevant for collisions contributing to the clock shift correspond to temperatures which may be significantly smaller than the temperature of the cloud at launch. Both the assumptions and predictions of our model are related to realistic parameters of operating caesium fountain clocks.

The operation of a caesium fountain primary frequency standard is greatly influenced by the characteristics of two other important facilities. The first is a stable frequency reference and the second is the frequency-transfer system. A stable frequency reference such as a hydrogen maser is a virtual necessity since essentially no fountain dead time can be tolerated without it. The frequency stability of this reference has a significant impact on the procedures for evaluating certain systematic biases in the fountain. State-of-the-art frequency transfer technology is also necessary if the fountain is intended to contribute to TAI or to be compared with other remotely located frequency standards without excessive degradation of stated uncertainties. We discuss the facilities available at the National Institute of Standards and Technology (NIST) and how they impact the operation of NIST-F1, the primary frequency standard at NIST.

We report on a new Kalman-filter hydrogen-maser time scale (i.e. JY1 time scale) designed at the National Institute of Standards and Technology (NIST). The JY1 time scale is composed of a few hydrogen masers and a commercial Cs clock. The Cs clock is used as a reference clock to ease operations with existing data. Unlike other time scales, the JY1 time scale uses three basic time-scale equations, instead of only one equation. Also, this time scale can detect a clock error (i.e. time error, frequency error, or frequency drift error) automatically. These features make the JY1 time scale stiff and less likely to be affected by an abnormal clock. Tests show that the JY1 time scale deviates from the UTC by less than ±5 ns for ~100 d, when the time scale is initially aligned to the UTC and then is completely free running. Once the time scale is steered to a Cs fountain, it can maintain the time with little error even if the Cs fountain stops working for tens of days. This can be helpful when we do not have a continuously operated fountain or when the continuously operated fountain accidentally stops, or when optical clocks run occasionally.

The NIM4 caesium fountain clock has been operating stably and sub-continually since August 2003. We present our improvements on NIM4 in 2005–06 and the most recent evaluation for its frequency shifts with an uncertainty of 5×10−15. A 203-day comparison between NIM4 and GPS time shows an agreement of 2×10−14. Finally the construction of the NIM5 transportable caesium fountain clock is briefly reported.

NPL operates a system of two primary caesium fountain clocks consisting of a fully characterised standard NPL-CsF2 together with a new standard NPL-CsF3, which has recently become operational. Both fountains feature a single-stage vapour-loaded magneto-optical trap as the source of cold atoms and an approximate cancellation of the potentially large cold collision frequency shift. As a result, the collision-shift type-B uncertainty is less than 10-16. Subsequently, more subtle systematic effects, including the frequency shifts from distributed cavity phase, microwave lensing and collisions with background gas have also been evaluated at the level of 10-16 or below. Now, as several systematic effects contribute to the fountains' uncertainty budgets similarly, further significant improvement of their accuracies is expected to be even more difficult. The short-term stability of these standards is also a significant factor limiting the overall precision as many days or even weeks of averaging is required for the type-A statistical uncertainty to approach the declared type-B systematic uncertainty. Going forward, further improvements in the reliability and robustness of operation of fountain standards is one of our priorities.

The cesium fountain primary frequency standard NIM5 started to operate since 2008 and started to report to BIPM since 2014. The major constrains of NIM5 is a relatively large background signal at the detection and microwave leakages due to the Ramsey cavity. A new fountain clock NIM6 is under construction. Besides some improvements on the vacuum system, a new Ramsey cavity and a microwave synthesizer are made to reduce the Type B uncertainty. Another feature of NIM6 is collecting atoms from a MOT loading optical molasses to get more atoms with a more uniform density distribution. With a new frequency synthesizer based on cryogenic sapphire oscillator (CSO), NIM6 is aiming to reach the quantum projection noise, thus leading to a reduced Type A uncertainty compared with NIM5.

Despite the rapid advances in optical frequency standards, caesium fountain clocks retain a critical role as the most accurate primary frequency standards available. At the National Research Council Canada, we are working to develop a second generation caesium fountain clock. Work is currently underway to improve several systems of FCs1, such as the laser system and microwave local oscillator, which will be incorporated into its refurbished version, FCs2. In addition, we have added an optical pumping stage which has increased the detected atom number by over a factor of six. In collaboration with the National Physical Laboratory (NPL), we are planning on replacing the physics package of FCs1. We will report on several recent improvements to FCs1, along with our progress in the development of FCs2.

The continuous cesium fountain clock FoCS-2 at METAS presents many unique characteristics and challenges in comparison with standard pulsed fountain clocks. For several years FoCS-2 was limited by an unexplained frequency sensitivity on the velocity of the atoms, in the range of 140 · 10-15. Recent experiments allowed us to identify the origin of this problem as undesirable microwave surface currents circulating on the shield of the coaxial cables that feed the microwave cavity. A strong reduction of this effect was obtained by adding microwave absorbing coatings on the coaxial cables and absorbers inside of the vacuum chamber. This breakthrough opens the door to a true metrological validation of the fountain. A series of simulation tools have already been developed and proved their efficiency in the evaluation of some of the uncertainties of the continuous fountain. With these recent improvements, we are confident in the future demonstration of an uncertainty budget at the 10-15 level and below.

We have realized a cesium atomic fountain using a novel launching method. We have obtained 108 atoms in the magneto-optical trap experiment and have studied the temperature and launching height with different laser frequency detuning to the cesium resonance line. A fountain with a height of 1.8 cm and a temperature of 30±10µK was achieved. In comparison with other experiments, we have also discussed the advantages and disadvantages of our method used in the present experiment.

We report a realization of Zacharias's 1953 proposal for observing a Ramsey resonance in an atomic fountain. Launched upward from a moving optical molasses where they have been cooled to ~ 5 ?K, cesium atoms pass once through a microwave cavity, continue to the summit of their trajectory, then fall again through the same cavity, completing the separated-fields interaction. The atoms spend 0.25 s in free flight above the cavity. Linewidth (2 Hz) and S/N imply a stability of 3?10-12 ?-1/2, at least as good as in existing Cs clocks, with eventual expected improvements of 102.

We report theoretical calculations of the transition probability errors introduced by microwave leakage in Cs fountain clocks, which will shift the clock frequency. The results show that the transition probability errors are affected by the Ramsey pulse amplitude, the relative phase between the Ramsey field and the leakage field, and the asymmetry of the leakage fields for the upward and downward passages. This effect is quite different for the leakage fields presenting below the Ramsey cavity and above the Ramsey cavity. The leakage-field-induced frequency shifts of the NIM5 fountain clock in different cases are measured. The results are consistent with the theoretical calculations, and give an accurate evaluation of the leakage-field-induced frequency shifts, as distinguished from other microwave-power-related effects for the first time.

The NIM5 fountain clock is the second fountain clock built at NIM (National Institute of Metrology, China), and has been operating stably and sub-continually since 2008. The fountain operates with a simple one-stage optical molasses to collect cold atoms, which reduces the collisional frequency shift dramatically. The fractional frequency uncertainty is estimated to be 2 × 10−15. The typical frequency instability of 2.5 × 10−14 is obtained at 10 s. Comparisons with other fountain frequency standards worldwide demonstrate agreement within the stated uncertainties.

The 21st International Conference on Spectral Line Shapes, ICSLS, was held in the historic main building of St Petersburg State University (St. Petersburg, Russia) on 3–9 June 2012. The event continued the tradition started in 1978 in Meudon Observatory in Paris. Representatives of line shape physics have since met every two years in different locations in Europe and North America. The most recent events were held in St John's, Newfoundland, Canada (2010), Valladolid, Spain (2008), and Auburn, AL (USA). Traditionally, the conferences consider experimental and theoretical issues of studying spectral line shapes, diagnostic utilization of spectral line profiles observed in absorption, emission or scattering of electromagnetic radiation by atoms, molecules, and clusters in different environments, including neutral environments, laboratory low and fusion plasmas, astrophysical conditions, and planetary atmospheres. The Conference was attended by over 100 professionals from Europe, Asia, America, Africa and New Zealand. The conference program was put together in such a way so as to exclude any parallel sessions. Five afternoon sessions featured 19 invited talks and 20 oral contributions, and two evening sessions offered 61 poster presentations, including post-deadline posters. This setup allowed for a relaxed and unhurried discussion of results and facilitated productive networking. The invited talks were selected by recommendation of members of the International Scientific Committee. The Organizers would like to thank all the members of the International Scientific Committee for their proposals on the agenda and their valuable advice. When considering candidates for oral contributions, the organizers took into account the suggestions and preferences of potential conference participants. When selecting the theses of poster presentations, the organizers focused on the topics in line with the theme of the conference and studies with well-formulated results. It must be noted that this year's conference included a noticeably larger number of reports on astrophysical applications and physics of planetary atmospheres. Another significant trend was related to a new area of inquiry: low temperature spectra, near-surface processes, and control and management of new technological processes. There were more presentations on elementary processes during the formation of spectral lines. Most of the presented studies were included in the Conference Proceedings publication. For the first time ever the proceedings will be published as a volume of Journal of Physics: Conference Series, published by the Institute of Physics (UK), making it possible to access the materials of the conference online. All published studies underwent peer review. The organizers would like to thank all the reviewers who found time to review the submissions against a very tight deadline during the summer. Professor N G Skvortsov, Vice-Rector for Research of St Petersburg State University, opened the conference. His speech was followed by a brief welcome message from Professor A Devdariani of St Petersburg State University, Deputy Chair of the Organizing Committee. An informal welcome party was held before the opening session. The cultural program of the event included various sightseeing tours, and a hydrofoil boat ride to the fountain city of Peterhof, where the delegates toured the park and attended a conference dinner. Organizers would like to express their appreciation to St Petersburg musicians, P Laul and D Kouzov for an excellent classical music concert. The conference included a meeting of the International Scientific Committee the minutes of the event are included in this volume (article number 011004). Most importantly, the next meeting, 22nd ICSLS, has been scheduled for 2014 in The Center for Laser Applications, The University of Tennessee Space Institute, US. Professor Christian Parriger will coordinate the organization. Organizers of the St Petersburg Conference would like to wish him and his colleagues every success for the next conference in Tennessee. The St Petersburg Conference was supported by the grants from St Petersburg State University, the Russian Foundation for Basic Research and Dynasty, Dmitry Zimin's Foundation for Non-Profit Programs. The Organizing Committee would like to thank them for their support. A word of thanks also goes to staff of the Research Department of St Petersburg State University for assistance with administrative issues, and to Mr S Slyusarev for creating the Conference logo and website. I would like to thank personally Professor A Kouzov, and Dr V Alexeev for providing much support in the process of preparing and holding the event in 2012. Alexander Z Devdariani St Petersburg University Sponsors St Petersburg University Dynasty Foundation Russian Foundation for Basic Research

Using modularized components, we have built a miniaturized optical system for 87Rb atomic fountain clock that is fitted on an 80 cm × 60 cm optical breadboard. Compared with the conventional optical setup on the table, our system is more compact, more robust and miniaturized. Taking advantage of this system, laser beams are transmitted through eight optical fibre patch cords from the optical breadboard to an ultra high vacuum system. This optical setup has operated for five months in our fountain system and required no alignment.

Two years ago John Kinchin and myself were lucky enough to attend the Goldsmith's particle physics course. As well as many interesting lectures and activities, this course included a visit to CERN. To most physics teachers CERN is Mecca, a hallowed place where gods manipulate and manufacture matter. The experience of being there was even better. Alison Wright was an enthusiastic and very knowledgeable host who ensured the visit went smoothly and we all learned a lot. While we were there, John and I discussed the possibility of bringing a party of A-level students to see real physics in action. In February of this year we managed it. 33 students from two schools, Boston Grammar School and Northampton School for Boys, and four staff left England and caught the 2 am ferry to France. Many hours and a few `short cuts' later we arrived at our hotel in St Genis, not far from CERN. The first day was spent sight-seeing in Lausanne and Geneva. The Olympic museum in Lausanne is well worth a visit. Unfortunately, the famous fountain in Geneva was turned off, but then you can't have everything. The following morning we turned up at CERN late due to the coach's brakes being iced up! We were met once again by Alison Wright who forgave us and introduced the visit by giving an excellent talk on CERN, its background and its reason for existing. At this point we met another member of our Goldsmith's course and his students so we joined forces once again. We then piled back into the coach to re-cross the border and visit ALEPH. ALEPH is a monster of a detector 150 m below ground. We divided into four groups, each with a very able and knowledgeable guide, and toured the site. The size and scale of the detector are awesome and the students were suitably impressed. We repeated the speed of sound experiment of two years ago at the bottom of a 150 m concrete shaft (320 m s-1), posed for a group photo in front of the detector (figure 1) and returned to the main site for lunch in the canteen. Over lunch we mixed with physicists of many different nationalities and backgrounds. Figure 1. In the afternoon we visited Microcosm, the CERN visitors centre, and the LEP control room and also the SPS. Here the students learned new applications for much of the physics of standing waves and resonance that they had been taught in the classroom. Later that night, we visited a bowling alley where momentum and collision theory were put into practice. The following morning we returned to CERN and visited the large magnet testing facility. Here again physics was brought to life. We saw superconducting magnets being assembled and tested and the students gained a real appreciation of the problems and principles involved. The afternoon was rounded off by a visit to a science museum in Geneva - well worth a visit, as some of us still use some of the apparatus on display. Friday was our last full day so we visited Chamonix in the northern Alps. In the morning, we ascended the Aiguille de Midi - by cable car. Twenty minutes and 3842 m later we emerged into 50 km h-1 winds and -10 °C temperature, not counting the -10 °C wind chill factor. A crisp packet provided an unusual demonstration of the effects of air pressure (figure 2). Figure 2. The views from the summit were very spectacular though a few people experienced mild altitude sickness. That afternoon the party went to the Mer de Glace. Being inside a 3 million year-old structure moving down a mountain at 3 cm per day was an interesting experience, as was a tot of whisky with 3 million year-old water. Once again the local scenery was very photogenic and the click and whirr of cameras was a constant background noise. Saturday morning saw an early start for the long drive home. Most students - and some staff - took the opportunity to catch up on their sleep. Thanks are due to many people without whom the trip would never have taken place. Anne Craige, Stuart Williams, Christine Sutton and Andrew Morrison of PPARC, but most especially Alison Wright of CERN and John Kinchin of Boston Grammar School who did all the hard work and organization. The week gave students a unique chance to see the principles of physics being applied in many different ways and I am sure this has reinforced their knowledge and understanding. Some students also took the opportunity to practise their language skills. The only remaining question is: what next? I'll have to think about it in the summer when I have some slack time. Hmm, SLAC, that gives me an idea....

A fountain atomic clock based on cold 87 Rb atoms has been in operation in our laboratory for several months. We therefore report the design of the rubidium fountain clock including its physical package, optical system and daily operation. Ramsey fringes have been attained with the signal to noise ratio of about 100.

Researchers at Northwestern University in Illinois have observed something truly remarkable by gently stirring a tank of fluid – an experiment that seems almost simple enough to set up in your own kitchen. After creating a vortex by spinning a disc in the fluid, Julio Ottino and colleagues injected some fluorescing dye, illuminated the fluid with laser light, and then saw an intriguing pattern of intricately ordered and chaotic structures floating before their eyes (G O Fountain et al. 1998 Science 281 683).

We developed a magnetic shield and generated a uniform magnetic field for an atomic fountain time and frequency standard at the National Research Laboratory of Metrology in Japan. The shielding factor was 8000 and the uniformity of the generated magnetic field was 0.2%, both of which are sufficient for the magnetic field to realize a required accuracy of 1 ×10-16.

By H. Baines London: Fountain Press. Pp. 320. Price 38s. 6d. It is to be hoped that the announcement on the dust cover that this book is "written for the layman" will not cause scientists to dismiss it as unworthy of their attention. True, it is made intelligible to the keen layman and the ordinary photographer by the inclusion of simple lessons in elementary science, but these are clearly marked and can be passed over by those with a basic scientific training.