ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 691 



object-slide ; R the proposed reflector. It is a section of a sphere. 

 The upper plane surface is to be brought in contact with the slide by 

 means of a suitable fluid in the usual way. The under surface is 

 concave. The dotted lines show the direction of the light, which 



Fig. 132. 



undergoes an inner total reflection at the surface C. Perhaps this 

 reflector will answer for the next limited period ; and when even this 

 shall prove to be insufficient, I propose to mount the object on the 

 plane surface of this reflector. In this way the theoretical limit 

 would be reached, and opticians can go on constructing objectives 

 that will take and utilize the oblique light of this reflector." 



[It is unnecessary to provide for any apertures in excess of 1 ■ 45 ; 

 and the assumption of y 1 ^- inch for the thickness of the object-slide 

 is unnecessarily large, ^ inch being the average. With these 

 alterations the maximum figures given by Mr. Gundlach ( T f inch 

 for the front lens and 3*98 inch for the back lens) would be 

 reduced. — Ed.] 



Symmetrical Illumination.* — Mr. Gundlach also desires " to call 

 attention to another idea, which, if carried out properly, may be of 

 advantage. I thought that a good result would be obtained if the 

 object should be obliquely illuminated symmetrically, i. e. from 

 diametrically opposite sides at the same time, with equal obliquity, 

 intensity, and quantity, rather than from one side only ; for the 

 secondary spectrum, with the unavoidable slight chromatic over-cor- 

 rection 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 resolving 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 -^^ of an inch 

 at a full turn of the screw ; for apparently the two images, projected 

 separately by the illumination 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 requires 

 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 



* Araer. Mon. Micr. Journ., iii. (18S2) p. 88. 



