J. D. HARDY ON FHOTO-iMICROGRAPHIC CAMERA. 307 



taken out, the cone of the camera is inserted easily but firmly 

 in the tube of the microscope, care being taken that the plate- 

 holder is parallel with the stage. Focus with an old thin piece 

 of sensitive plate, which has been "exposed" and developed, 

 with a fairly transparent film (any ground glass is much too 

 coarse for the purpose). This is cut to size so as to drop easily 

 on to the plate-holder ; the film side downward, so as to corres- 

 pond with the position of the sensitive plate exactly. Focussing 

 can be done with an ordinary watchmaker's lens. Having 

 focussed the object on the focussing plate, remove it carefully, 

 turn down the lamp as low as possible, and drop a piece of 

 sensitive plate on to the plate-holder ; put on the lid and turn 

 up the light. Exposure is a matter of experience, as objects 

 differ in photographic quality according to density or colour. 

 The O.G. power has also to be considered. As a time direction 

 I find that with a good lamp light and 1-inch O.G. from one- 

 and-a-half to tw^o minutes is suflQcient. After exposure the light 

 must be again turned down and the plate removed to dark box. 

 Unless it can be developed at once, two or three exposures 

 should be made with different times. If you do not do the 

 developing yourself any photographer would do it, care being 

 taken to let him know the class of object and what to look for ; 

 but this and other matters are within the experiences of photo- 

 graphy. 



The flange is for the purpose of inserting a dark slide when 

 the light cannot be turned down. It is one of Marion's metal 

 slides, which are sold with their Academy camera, of about 2in. 

 square. As it is very convenient to have one or more of these 

 they should be obtained previously to making the camera, so 

 that the flanges may be made to fit easily. When focussing for 

 this dark slide a thicker piece of plate should be used, the film 

 side uppermost, so as to correspond with the plane of the plate 

 in the slide. 



The aim of the whole is to make an instrument practically 

 useful, sufficiently light so as not to disturb the position of the 

 microscope, and also to place the sensitive film in exactly the 

 same plane as the focussing plate. 



Mr. Hardy afterwards exhibited the negative of a diatom 

 (Coscinodiscus excavatus), showing the markings on the secondary 

 film and of lin. diameter. 



