168 



THE AMERICAN MONTHLY 



[September, 



Optical Arrangements for Photo- 

 micrography, and Remarks on 

 Magnification.* 



BY ROMYN HITCHCOCK. 



There are two methods of obtain- 

 ing ampHfication in photographing 

 microscopic objects ; one is by regu- 

 hiting the distance of the sensitive 

 plate, the other is by the interposition 

 of an eye-piece, or a supplementary 

 lens, usually an acromatic concave, 

 between the objective and the sensi- 

 tive plate. It is the relative merits 

 of these two methods that we pro- 

 pose to briefly discuss. 



The fact that both methods are used 

 by different persons with perfectly 

 satisfactory results might lead to the 

 inference that it is purely a matter of 

 convenience. This, however, is not 

 the case ; for the ordinary eye-pieces 

 are not very perfect optical instru- 

 ments, and it can scarcely be sup- 

 posed that they will preserve the per- 

 fection of an image formed by an 

 objective, in all its details. The con- 

 ditions in photography are somewhat 

 different from those of ordinary ob- 

 servation, wherein the defects of the 

 ocular are not noticeable. The eye 

 fails to discover a curvature of the 

 image which is very evident when the 

 latter is spread over a flat focussing 

 screen six or eight inches square. 

 In photography much depends upon 

 whether the operator uses large or 

 small plates, for with a small plate 

 giving a field of three or four inches 

 veiy high magnification can be satis- 

 factorily attained by the aid of an 

 ocular. For ordinary purposes such 

 plates are quite large enough, but 

 when we come to large sizes, such as 

 eight by ten-inch plates, the eye-piece 

 will not give sharp definition all over 

 the field. 



The eye-piece enables one to ob- 

 tain increased magnification with 

 short camera-bellows, and we have 

 seen small photographs of difficult 

 subjects taken with the eye-piece 

 which leave nothing to be desired. 



* Read before Section G, at the meetingof the A. A- 

 A.S., Ann Arbor, 1885, by Dr. H. G. Beyer, U, S. N. 



Nevertheless, for the more difficult 

 objects, and whenever finest details 

 are to be photographed, we would 

 not advise its use. By far the better 

 plan is to use a long camera -box, and 

 get amplification by increase of dis- 

 tance. In this way very good results 

 can be obtained, but it must be ob- 

 served that the objective requires to be 

 specially corrected for the distance of 

 the plate, and this is not only incon- 

 venient, but in many cases quite im- 

 practicable. Thus, in using an ob- 

 jective of the oil-immersion form 

 without any collar adjustment, one 

 can only focus the image on the plate 

 by causing the objective to approach 

 nearer to the object than where it is 

 used in ordinary observation. 



Another plan which has been highly 

 recommended by Dr. Woodward, Dr. 

 Van Heurck, and others, is to make 

 use of an amplifier. Dr. VanHeurck's 

 very ingenious device has been de- 

 scribed on page 45 of the current vol- 

 ume. Dr.* Woodward was the first 

 to point out the advantages of the 

 amplifier in photomicrography, and 

 also to give instructions for its proper 

 application. 



In working with objectives cor- 

 rected with the utmost care for a de- 

 finite length of tube, it is obvious 

 that -any change in the course of the 

 rays passing through the objective 

 will introduce aberrations which will 

 impair the definition. If we focus 

 upon an object with the ocular, then 

 remove the ocular and receive the 

 image upon a focussing screen sey- 

 eral feet away, the objective must be 

 moved nearer to the object in order 

 to give a sharp image on the screen. 

 Shortening the working focal length 

 in this wa}' obviously interferes with 

 the normal course of the rays through 

 the lens, and it is therefore not possi- 

 ble to obtain such a perfect image in 

 this way as may be seen with the 

 ocular. It will readily be seen that 

 the only way to secure the best defi- 

 nition is to focus the objective with 

 the ocular, for then we know the cor- 

 rections arc properly adjusted, and 



