88 



THE AMERICAN MONTHLY' 



[May, 



Symmetrical Illumination. — 

 Before concluding I desire to call 

 attention to another idea, which, if 

 carried out properly, may be of ad- 

 vantage. I thought that a good 

 result would be obtained if the object 

 should be obliquely illuminated sym- 

 metrically ; i, <?., from diametrically 

 opposite sides at the same time, with 

 equal obliquity, intensity and quan- 

 tity, rather than from one side only ; 

 for, the secondary spectrum, with the 

 unavoidable slight chromatic over- 

 correction of the outer part of the 

 objective, produces a more or less 

 visible and disturbing spectrum, 

 which will be neutralized in the pro- 

 posed way. I have tried this ; and, 

 after some difficulty, I think I suc- 

 ceeded in obtaining a result in re- 

 solving which I could not get in the 

 usual way. 



From my limited experience in this 

 matter, I can say, however, that this 

 symmetrical illumination requires a 

 very delicate fine-adjustment ; the 

 one I used gives a motion of only 

 g-J^j- of an inch at a full turn of the 

 screw ; for, apparently, the two ima- 

 ges, projected separately by the il- 

 lumination from each side, do not 

 move in the direction of the optical 

 axis when the screw is turned ; but 

 they move each toward the side from 

 which they are projected, and it re- 

 quires great'precision to get them to 

 coincide perfectly. Further desirable 

 experimenting in this, for which I do 

 not deem myself competent, I feel 

 obliged to leave to experienced and 

 skilful microscopists ; and I shall be 

 grateful if informed of the results of 

 any experiments tried by them. 



Photograpliing with the Mi- 

 croscope*. 



For those persons who, like myself, 

 are not skilled in the use of the pen- 

 cil, a simple and inexpensive process 

 of photographing microscopic objects 

 is of great value. I have long felt the 



* Read by the Editor before the New York 

 Microscopical Society, May 5th, 1882. 



need of an apparatus for this purpose, 

 and some time ago I prepared a dark 

 room in which I could use the ordin- 

 ary collodion process with sunlight. 

 But unless one can set apart a room 

 for such work, and can afford to 

 spend a considerable amount of 

 money for a good heliostat, and for 

 the necessary apparatus and chemi- 

 cals, there is but little pleasure in 

 working with wet collodion plates. 

 I have, at the present time, a long 

 camera, specially arranged for photo- 

 graphy with sunlight, which enables 

 me to work without a dark room, but 

 since I have not an entire house to 

 myself, J cannot find room for it, and 

 it is never used. Within the last few 

 years, a more simple process of pho- 

 tography has been perfected, which 

 is now very popular among amateurs 

 for landscape views. This is the dry- 

 plate process ; and it is the applica- 

 tion of this process to photomicro- 

 graphy that I desire to describe this 

 evening. If my attempt is success- 

 ful, I shall have the pleasure of pre- 

 paring and finishing a negative before 

 the close of the meeting. 



Let me first describe a very simple 

 form of apparatus which, while it 

 meets all requirements, is quite inex- 

 pensive. I have here an ordinary 

 camera-box with bellows (fig. 30). 



Fig. 30. 



The apparatus consists of a board, 33 

 inches long, and as wide as the came- 

 ra. At one end is placed the camera, 

 which is fixed in position by cleats, 

 so that it cannot be moved in either 

 direction, although it can readily be 

 lifted up from the board. In front of 

 the camera the microscope, arranged 

 horizontally, is placed with the end 



