90 



THE AMERICAN MONTHLY 



[May, 



to look through the tube of the mi- 

 croscope, place the light in the axis. 

 Then arrange the condensers, put on 

 the object and the lenses, and focus 

 on the ground glass. The focussing 

 may be done by a focussing-rod run- 

 ning from the back of the camera and 

 acting upon the fine-adjustment of 

 the microscope. This is a necessary 

 arrangement when the observer can- 

 not reach the focussing-screw while 

 looking at the ground glass. But I 

 have not found it necessary with this 

 apparatus, for I can reach the fine- 

 adjustment quite readily. 



When the image is distinct upon 

 the ground glass, the fine-adjustment 

 should be moved so as to withdraw 

 the objective more or less from the 

 object, to focus the chemical rays. 

 Just how much the tube must be 

 withdrav/n must be determined for 

 each objective. 



When everything is properly ar- 

 ranged, the sensitized plate, which is 

 in the plate-holder, all ready, is placed 

 in position. The plate-holder may 

 be constructed to hold either one or 

 two plates. For microscopical work 

 one holding a single plate is sufficient, 

 and such a holder is the cheaper. 



The sensitized plates used in the 

 dry-plate process can be purchased in 

 packages of a dozen, and they are 

 not very costly. The camera I am 

 using is made to take plates five 

 inches wide by eight in length, and such 

 plates cost $i.8o per dozen. This 

 size, however, is not suitable for mi- 

 croscopical work. I have made a 

 frame for my plate-holder which en- 

 ables me to use plates five inches by 

 four, which only cost $0.95 per 

 dozen, and these are large enough for 

 ordinary work. A square plate would, 

 of course, be better, but square plates 

 are not used by photographers, and 

 therefore cannot be found in the 

 market. I could easily arrange to use 

 a plate five inches square in my 

 holder. Mr. Adams, the manager of 

 the Scovill Manufacturing Company, 

 has taken some interest in the subject 

 of microscopical photography ; and I 



have been trying to induce him to 

 manufacture a camera especially 

 adapted to microscopical photo- 

 graphy. If such a camera is made, as 

 I have no doubt it will be soon, it 

 should have a longer bellows than 

 the one before you, and square plates 

 should be used. I think plates five 

 inches square would be the most 

 satisfactory size for general use. 



The plates are sensitized by coating 

 them with what is termed a gelatin 

 emulsion. Without giving the details 

 for the preparation of the emulsion, 

 I will say that it consists of a solution 

 of gelatin holding in suspension min- 

 ute particles of silver bromide. The 

 particles of bromide are so minute 

 that they can hardly be seen by the 

 aid of a microscope. When a thin 

 film of the emulsion is dried upon the 

 glass plate, the plate is extremely 

 sensitive to light. Wherever a ray 

 of light strikes the plate, a molecular 

 change is produced in the particles of 

 silver bromide. When the plate is 

 exposed in the camera, the image of 

 the object to be photographed is im- 

 pressed upon the bromide by the 

 action of the light, and the amount of 

 change is proportional to the intensity 

 of the light. Hence, where there is an 

 image of a white object, or, what in 

 microscopical work with transmitted 

 light amounts to the same thing, 

 where the light passes to the plate 

 without passing through the object, 

 the change is greatest, and with every 

 different shade there is a proportional 

 difference in the action on the plate. 

 Opaque structures, or those which 

 transmit only red or yellow rays, pre- 

 vent any change in the sensitive 

 surface. 



But after the plate has thus been 

 acted upon by the light, whatever 

 change has taken place is quite invisi- 

 ble. The latent image will remain in- 

 visible for an indefinite time, if the 

 plate is preserved in darkness. To 

 make it visible it must be devel- 

 oped ; and although this operation 

 should be done in the dark room, 

 lighted by a lantern with ruby glass, 



