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A Wilson cloud chamber 22 inches in diameter in a magnet producing a magnetic field of 21,700 gauss at the center of the chamber is described. The magnet is pulsed in synchronism with the 184‐inch Berkeley cyclotron and can be operated with a steady field of 10,000 gauss. The use of the instrument for identifying light atoms is discussed briefly.

A diffusion cloud chamber 8 ft long, 4 ft wide, and 5 in. deep has been built and operated successfully for 540 hrs out of a continuous 720‐hr run. Though the actual area is 32 sq ft, the area photographed was 21 sq ft or approximately 2 sq m. A sensitive layer of good quality extending 2½ to 3 in. above the floor was easily maintained. The coolant temperature was held near −67°C and the air above the glass sheet forming the top was held at 30°C. A clearing field of 30 v (top negative) was found desirable to keep the chamber free from general rain. The chamber contained an air and methyl‐alcohol mixture operated at 1 atmos pressure. Tracks in the sensitive region were photographed through the top with the aid of several mirrors. The chamber was used to study the structure of the cores of cosmic‐ray air showers. It is now being used for studies with the bevatron.

An experimental measurement of the angular distribution of protons scattered by neutrons in the neutron beam of the 184-inch Berkeley cyclotron has been made with a Wilson cloud chamber with magnetic field. The results show that the scattering is not isotropic in the center of mass system and that it is not symmetric about 90 degrees. The peak of protons in the forward direction indicates that a certain amount of charge exchange is taking place between the neutron and proton.

Bright‐field illumination using a beaded reflective screen is accomplished in the 30‐in. heavy liquid bubble chamber by using separate light sources for each lens. The resolution obtainable is about four times as great as with dark‐field illumination.

A study has been made of eighty-two stars produced in a mixture of oxygen and helium when a cloud chamber filled with these gases and placed in a magnetic field of 13,000 gauss was bombarded by 90-Mev neutrons. Stars having up to five prongs were observed.Of the forty-eight 2-prong stars, 14 could be reasonably assigned to oxygen and 14 to helium. The remaining 20 were unidentified. It was found possible to make plausible identifications of all the fragments from only 7 helium stars and one 5-prong oxygen star. The general identification procedure is discussed and the arguments for the one 5-prong oxygen star are presented in some detail. Histograms of the ${H}_{\ensuremath{\rho}}$- and scatter-angle distributions of the star fragments are given.

A 36‐atmosphere diffusion cloud chamber has been constructed for use with the 184‐inch cyclotron and with the bevatron. Minimum‐ionizing particles leave dense, sharp tracks of good contrast with hydrogen filling at design pressure. In the operation described, one event is obtained every 15 minutes for a reaction cross section of 30 mb for negative pions on hydrogen. Among the unique features of the chamber are (a) the high pressure; (b) the high magnetic field; (c) the simple unit preassembly of the chamber and the upper pole piece; (d) the distortion‐free photography afforded by the small camera windows of good optical quality directly in front of the camera lenses and the absence of a large top glass near the sensitive region; (e) the thin window for collimated beams; (f) the supported cast Astrolite windows for illumination; and (g) the satisfactory sensitive layer obtained despite the large height‐diameter ratio of the pressure vessel.