ON PHOTO-MICROGRAPHY. 185 



the tube when at its shortest, shall just touch the 

 edges of all these apertures. Thus arranged they 

 will not contract the field, and will not allow a ray 

 of light to fall on anything but the front faces of 

 the diaphragms themselves, whence it cannot be 

 reflected to the plate. \Vhen the eye-piece is used 

 the diaphragms must be differently adjusted, for 

 the tube then practically ends at the front surface 

 of the field glass, and its diameter is practically 

 the clear aperture of that lens. A slight sliding 

 backward or forward of the diaphragms will be 

 sufficient to effect this adjustment. In the camera 

 itself the diaphragms, if required, will take the 

 form of sheets of blackened card with central aper- 

 tures. The writer, who works with quarter plates 

 (4J- by 3|- inches), prefers to make the aperture 

 nearest the microscope small and circular, the next 

 larger and shortly oblong, with very rounded 

 corners, the next more oblong and with less rounded 

 corners, and the last oblong with acute corners and 

 the shape of the plate, but a trifle smaller. On 

 looking through the whole length of over a yard of 

 camera and tubes when so arranged, and with a 

 blaze of light streaming in from the condensers, 

 not a single stray beam can be seen, and nothing is 

 visible but the object glass " full of light." Then 

 when a plate is properly exposed and developed, 

 the deepest shadows come out as clear as the glass 

 itself, and the negative prints brightly and quickly. 

 When working with 3, 2, or even 1 inch objectives, 

 the fine adjustment may be dispensed with, and 

 then, if the microscope be of the old Ross pattern, 

 the tube may be entirely discarded, and the objec- 

 tive screwed (by means of a short adapter, if need 



