The Photographic Society

refractive index. If two crystals of a similar refractive index are placed under an ordinary microscope they are indistinguishable, but when polarised light is employed they show up in different colours.

Mr. Hall brought along many interesting exhibits, including some excellent photographs, and at the end of his very intensive and absorbing lecture a recently produced film on the polarising microscope was shown.

Manor produced two lectures for the final meeting of the term. The first, "Ramming Home the Charge", was given by the Secretary. This was an explanation of the Induction Power Impulses and Megaphone Exhaust Systems as employed on a racing motor. He explained how use was made of two natural phenomena connected with gases moving in a tube, and how, as a result of these conflicting requirements, calculations concerning them were extremely complicated. In conclusion he pointed out a few of the difficulties which occurred on the practical side. "Explosives in Industry" was the title of the second lecture, given by Wilson, R. B. He described how some of the more common explosives were manufactured, such as nitroglycerine. This in a pure liquid state is unsuitable for general use owing to its sensitivity to mechanical shocks. When it is mixed with "kieselguhr" (a chalk-like material), it is known as dynamite, and it is also employed in the manufacture of cordite. He went on to describe different kinds of detonators, and showed how a fuse lead should be connected, after which he set it off. The fuse burns at the astonishing rate of seven miles per second.

After being shown some photographs of different methods employed in breaking up rocks the "Society" moved outside to see a demonstration. The third "bang" produced was so violent that there were even murmurs of history repeating itself, but as far as is known the rumour that the Guard at the Houses of Parliament has been reinforced has no foundation. J.F.

Chairman: MR. E. K. ROBINSON. Secretary: D. A. STABLER.

Although this term's activities have been quieter than last, there has been no lack of enthusiasm among the members. Two lectures, with demonstrations, have been held : "Colour Transparencies" by J. Saville, and "Retouching" by B. N. Jackson. Both were excellently delivered.

Three tickets have again been bought from the York Camera Club, and members have again enjoyed the Thursday evening meetings. 32