10 PHOTOMICROGRAPHY 



adjustment, but no mechanical focussing device gives 

 quite such delicate control over the fine adjustment as 

 is obtained by direct manipulation of the milled head. 

 Other focussing arrangements are shown at Figs. 41 and 

 46, and a well-known device for the purpose is a Hooke's 

 handle (Fig. 67). A little play must be allowed in the gear 

 wheels connecting an unsupported handle to the fine 

 adjustment, otherwise the focus will be upset when the 

 rod is laid down. 



Screen for Light Adjustment. An easel covered with 

 white paper forms part of the apparatus shown in 

 Fig. 2. The camera can be removed and replaced by this 

 screen for the preliminary adjustment of the light, as it is 

 easier to detect uneven illumination on the white surface 

 than on the ground glass. The image is also more readily 

 examined and focussed on the white paper, and it is 

 possible to place the focal plane of the camera exactly 

 in the position originally occupied by the screen by means 

 of a scale fitted along the baseboard, and pointers on the 

 camera back and easel. The scale is also of use for deter- 

 mining by calculation from the camera extension the 

 magnification produced by any objective and eyepiece 

 whose powers are known. 



Light-tight Connection between Microscope and Camera. 

 The connection between the camera and microscope 

 must be light-tight. A velvet sleeve with an india- 

 rubber ring sewn into one end to fit over the eyepiece, 

 and the other end joined to the camera, answers the 

 purpose very well ; or a metal or cardboard ring about 

 an inch deep can be fitted into the lens aperture 

 of the camera, and a cell into which it slips placed 

 over the eyepiece. One of these cells is shown in 

 Fig. 1. 



Focussing Screens. The ground glass fitted to the 

 camera back for focussing purposes should be as fine as 

 possible, in order that delicate structures may be made 



