228 SCIENCE PROGRESS 



The illuminating apparatus used is an optical bench, 

 designed for use with either ordinary or monochromatic 

 light. This was originally designed by Dr. E. H. Spitta for 

 use with the electric arc. His plan was then modified and 

 simplified by Mr. H. G. Plimmer, so as to be available for the 

 Nernst lamp, and it is on this latter model that the present 

 arrangement is founded. The lamp, of one vertical filament, 

 is clamped at one end of the optical bench. An achromatic 

 condenser focusses the image of the filament on an adjustable 

 slit; a collimating lens collects the light from this and emits 

 it in a parallel bundle to a narrow-angle prism, which has a 

 Thorpe's diffraction grating on one surface, transmitting an 

 approximately normal spectrum. This is focussed by a third 

 lens on the substage of the microscope. By rotating the 

 apparatus any part of the spectrum can be employed, and the 

 microscope slide illuminated by light of any desired wave- 

 length with sufficient accuracy for the purpose required. 



This can be used in two ways. One can, by taking only 

 the violet end of the spectrum, obtain the highest resolution 

 possible in visual work. But for the purpose of photography 

 this use must generally be subordinated to the necessity of 

 obtaining sufficient contrast in the negative. The latter effect 

 can be obtained without any calculations in a very ready and 

 simple manner by rotating the illuminating apparatus, and at 

 the same time observing the ground glass of the camera. At 

 a certain colour the best contrast for the particular microscopic 

 stain under examination is reached, and as the light is mono- 

 chromatic the photograph exactly reproduces the visual appear- 

 ance. The amount of time saved by this simple method will 

 be best appreciated by those who have struggled with the older 

 devices. 



The time of exposure is less than one would have supposed, 

 varying from 20 seconds at 100 diameters to about 10 — 15 

 minutes at 1,000. It can be shortened if the Nernst filament be 

 placed at the focus of the collimating lens and the slit removed, 

 but this is very seldom worth doing. 



Photomicrography is with most people only a means to an 

 end — namely, the production of photographs of the highest 

 quality in the shortest time, and as the apparatus described has 

 been found to fulfil these requirements it is hoped that the 

 description will be found of use to other workers. 



