400 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 3. 



Fig. 4. 



Part of head of Telescope fly, Diopsis apicalis x 22. 



Fig. 3. " M " plate, C filter, to obtain maximum contrast. 

 Fig. 4. " M " plate, F filter, to obtain maximum detail. 



which does not transmit any red, especially for visual use when the 

 highest possible resolution is required. They invite inquiries for it 

 under reference No. 38aM. A. N. D. 



Telescopic Focussing Apparatus for Photomicrography. — A. F. 

 Hallimond, M.A. {Iron and Steel Institute, 1918). In the ordinary 

 vertical camera, in which the plate is supported at a distance of 10 in. 

 or more above the microscope, the height of the focussing screen is 

 often the source of much inconvenience. It was suggested to the 

 author by Mr. J. H. Whiteley that to meet these difficulties a means 

 might be found of reflecting the beam horizontally, and so adjusting 

 the focus without the use of a ground-glass in the usual position. The 

 telescopic arrangement here described was designed for this purpose, 

 and resembles the ordinary form of reflex focussing camera. 



The construction of the apparatus is shown in the adjoining figure, 

 where the broken lines indicate the path of a beam which converges to 

 form a point in the image. • 



Rays proceeding from the microscope eye-piece A, which would 

 normally converge to a focus on the photographic plate, are reflected 

 horizontally by the movable mirror B ; this mirror is attached to a 

 metal plate large enough to cut off all light from the camera when in 

 the position shown, and is prevented from passing beyond the 45° 

 position by an adjustable stop D attached to the spindle. The deflected 

 beam is focussed by the telescope objective E (focal length 4 in.) upon 

 the cross-wires F, and the image so formed is seen, together with the 

 cross-wires, when examined through the eye-lens G (focal length f in.). 

 The magnification thus obtained is about the same as that given by the 

 use of a hand-focussing lens upon (or without) the ground-glass. 



It will be seen that to each length of camera there corresponds a 

 fixed position of the focus at F, to which the cross-wires must be set by 



