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Laser physics can be addressed and presented at three different levels of conceptual and mathematical sophistication. In this paper, the three levels are defined and characterized. Advantages and disadvantages of each approach are discussed, and recommendations for choosing a particular approach to match student level and interest are offered. (Author/DT)

This paper describes the instrumentation and operation of a photon counting system which was constructed to measure the delayed light emission from the algae Scenedesmus . The functioning of the system was illustrated by a plot which compared data of the recorded delayed light emission with data of the system noise over the first 20-μsec interval after extinction of a modulated laser beam. We believe that this paper shows the utility of photon counting as a satisfactory method of measuring biological light emission, especially a very rapid process such as the delayed light emission from photosynthetic organisms.

The earliest recorded study of the transmission of light by the ocular *media is attributed to Aschkinass (1895), but according to Meyer-Schwickerath (I960) his method was grossly inaccurate. Luckiesh (I92 I) calculated the transmissivity of the ocular media on the basis that it equalled that of a layer of water 2-28 cm. thick. Shoji (1923) and Bucklers (I926) both made measurements of transmission but the wavelengths used were all below 400 m,u. In a classic paper, Ludvigh and McCarthy (I938) estimated the transmission of light of wavelengths from 400 to 820 mg to be 50 per cent. This paper is much quoted in the subsequent literature, and Davson (I963) quotes only the figure of 50 per cent. In fact Ludvigh and McCarthy based their estimate on experiments on only four eyes, all of which had been enucleated because they contained malignant melanomas. It can be reasonably argued that the ocular media may well have undergone pathological change while remaining clear to the naked eye. The results of Ludvigh and McCarthy (1938) appear to have been accepted until 1951, when they were seriously questioned by Wald (I 95 I). At that time Meyer-Schwickerath (I95I), in developing his light-coagulator, became interested in the transmission of light by the ocular media; he repeated the experiments of Ludvigh and McCarthy (I938) and arrived at the same value of 50 per cent. However, he had no hesitation in discarding the results because the specimens used showed signs of post mortem change when examined critically. Weale (I953), discussing the nature of light reaching the retinal elements after its passage through the ocular media, stated that "as far as the human eye is concerned, such knowledge is very inadequate in the case of the enucleated eye and non-existent in that of the eye in situ". Meyer-Schwickerath (I954) carried out further studies and calculated absorption values on the assumption that they were the same as that of 2 28 cm. water, quoting Goldmann, Konig, and Mader (I950) to justify this assumption. Such calculations showed that, at a wavelength of 694-3 mg, transmission via the ocular media was about 95 per cent. Wiesinger, Schmidt, Williams, Tiller, Ruffin, Guerry, and Ham (1956) studied the transmission of light via the ocular media of nine freshly-enucleated rabbit eyes, using a spectrophotometer operating from 380 to I,400 mg±. Their results also showed that, in the 694-3 mg range, transmission is of the order of 95 per cent. The technique appears to be free of major sources of error but, again, all the ocular media were grouped together in this study. There seems to be no work recorded in the literature in which the transmission of non-coherent light through the individual ocular media has been measured. Ham, Williams, Geeraets, Ruffin, and Mueller (I963) and Geeraets and Berry (I968) confirmed the figure of 95 per cent., again using non-coherent light. It may be concluded that the

Coherent optical signal processing techniques applied to the screening of exfoliated cytologic cervical samples were evaluated. The two-dimensional Fourier spectrum of some 80 isolated cells was obtained from high resolution cell photographs and recorded. Quantitative spectrum data were collected on a training set consisting of 15 categorized malignant cells taken from slides diagnosed as malignant and 10 categorized normal cells taken from slides diagnosed as normal. A variety of transform parameters were measured and certain combinations of them were found which separated completely the normal from malignant cells. Certain parameters were shown to be functionally related to the cell diameter, nuclear diameter and nuclear density. Other parameters were found which appear to be related to other cell features such as clumping of nuclear deoxyribonucleic acid. When they were used in combination with the previous parameters, they greatly enhanced the discriminatory capabilities. Computer model studies formed the basis for our experimental design and parameter selection and have validated much of our experimental results. Although the sample size to date is admittedly small, our results have been very encouraging. The purpose of this paper is to report on the initial results obtained in a study to determine the effectiveness of coherent optical processing techniques when applied to the screening of’ exfoliated cervical cytologic samples for malignancy. These techniques rely on the diffraction properties of coherent light which are closely related to scattering techniques used by the groups at the Lawrence Livermore Laboratories and the Max-Planck Institute for cytologic screening and sample enrichment. The specific approach used in this investigation is concerned with the relationship the cell image has to its two-dimensional Fourier transform, which is generated optically by a well known method described below. The success of the screening approach depends upon the ability to measure and discriminate between the Fourier spectrum of malignant and Fourier spectrum of normal exfoliated cervical cells. In most instances the measured discriminating parameters can be functionally related to well known cytologic characteristics which are conventionally used to discriminate between malignant and normal exfoliated cervical cells.

This paper reports the first Raman spectroscopic study of the potassium complex of the cation-specific antibiotic valinomycin. Complete Raman spectra (140 to 3600 cm−1) of crystalline valinomycin-KSCN and its CCl4, CHCl3 and C2H5OH solutions are presented and used to probe the structure of the complex in these environments. In all cases a single, narrow peak is observed in the ester CO stretch region (1750 to 1775 cm−1) which contrasts strongly with the broad bands observed in solutions of uncomplexed valinomycin. This is consistent with the presence of a single conformation in which all six ester CO groups co-ordinate an enclosed potassium ion. We find that although the ester CO stretch frequencies of the complex are similar in the solid state and in non-polar solution (~1770 cm−1) they are considerably different in the presence of polar solvents (~1756 cm−1); this may indicate that the complexed potassium ion is still free to interact with nearby solvent ions (and possibly its counterion) through gaps in the hydrophobic “shield” provided by the hydrocarbon residues of valinomycin. In contrast the amide CO frequencies of the complex (~1650 cm−1) are solvent-independent. These groups are apparently strongly hydrogen-bonded to provide a rather rigid, compact framework for the complex conformation.