THE MICROSCOPE. n 



use of direct sunlight with the central mirror, but taking the light 

 from the edge of the reflector, where the illumination was good, but 

 not intense. I found upon calculation, however, that this gave the 

 light an obliquity of about io°; and then, swinging the mirror io° to 

 one side, so as to bring the edge reflecting the light used directly in 

 the optical axis, I obtained still the same admirable results. I have 

 done this again and again without the slightest difficulty, and shown 

 the diatom to a considerable number, who recognized the transverse 

 lines without any difficulty whatever. 



Sincerely yours, 



S. A. Forbes. 



Champaign, 111., March 12, 1S83. 

 B. W. Thomas, Esq., 27 Portland Block Chicago, III.: 



My Dear Sir: — Please inform our friends that I have suc- 

 ceeded in finding out what reflects the light in resolving amphi- 

 pleura by sunlight, mirror central. It is nothing but the immersion 

 (homog.) fluid, which, if extending to epodisc argus, acts like a lens 

 above the object. The nearer perfect the lens, the better the resolu- 

 tion. 



Yours very respectfully, 



H. J. Detmers. 



Office of State Entomologist, 

 Normal, 111., March 17, 1883. 



1 



Dear Mr. Thomas — I have just tried your experiment with 

 the diaphragm, varying it, however, by darkening the mirror instead 

 of using two preparations. When the mirror is all darkened, except 

 a circular area 4 mm. in diameter, directly sub-central, the resolu- 

 tion is still perfect. I have had the objective loaded at the same 

 time to cut off light from above. There is no question that the 

 light is directly central until it reaches the slide. What becomes of 

 it then is another question which I haven't time to work at. I think 

 it possible, however, that some of the rays outside the field of the 

 microscope get reflected back and forth between the glass surfaces, 

 and finally reach the object. As the light comes from a concave 

 surface, many of these rays must be slightly oblique to the slide, no 

 matter how small a part of the mirror is used, and a very slight 

 obliquity would be sufficient to start these reflections. 



