PRACTICAL PHOTO-MICEOGBAPHY. 53 



sufficient grain to show an image. A sensitive gelatine plate 

 exposed for a second to light, developed, fixed, washed and 

 slightly treated with mercuric bichloride gives a good surface; 

 this, we believe, orginated with Mr. Walmsley. No better 

 focusing surface will be found than a piece of glass with some 

 diamond marks on the front that is on the side next the 

 light. In the writer's case these marks consist of a cross, the 

 arms of which are inches divided into tenths with a diamond ; 

 when viewing the image with the focusing eye-piece, if the 

 magnification be known, it is easy to measure objects and dis- 

 tances at a glance. No image can be seen on the plain glass 

 unless an eye-piece be used to focus the aerial image ; the glass 

 in fact is used only as a rest for the focusing eye-piece. The 

 eye-piece used is known as a Kamsden, or perhaps better a 

 Zeiss " aplanatic magnifier " may be used. In either case the 

 scratches on the front of the focusing glass must be most care- 

 fully focused with the eye-piece ; if is found difficult to focus 

 the scratches, a fly's wing or some such object may be fixed to 

 the front of the glass plate and the magnifier set to focus on 

 that. It goes without saying that the focusing screen or 

 ruled glass or whatever it is should be in the same plane as 

 that occupied later by the sensitive plate. The rays, however, 

 in ordinary work are at the sensitive plate so nearly parallel 

 that slight " want of register " between plate and focus-screen 

 is not so very fatal as many think. Dr. Bousfield, whom no 

 one need fear to follow, uses no glass plate at all, nor fixes his 

 focusing eye-piece at any point as many do, but focuses the 

 aerial image in air alone; none the less the glass plate affords 

 a convenient rest for the Kamsden or Aplanatic, but there 

 must be no heavy pressure of focuser on screen. 



There are many other ways of producing a focusing screen 

 but probably the best have been here noticed. 



It is important to have an arrangement inside the camera for 

 starting and stopping the exposure. At " 7 " on Fig. 16 

 is seen the exterior of a simple flap shutter, the flap inside be- 

 ing operated by the button outside. Sometimes when a very 

 rapid exposure is required and when consequently there is fear 

 of moving the whole apparatus, we close this flap, open the dark 



