184 THE AMERICAN MONTHLY [June, 



nothing at the present time as it does not bear directly 

 on the matter at issue. But observing that hitherto all 

 microscope objectives had been so corrected that the 

 chemical and visual foci did not correspond in position 

 and that if I wished to take photographs with the lenses 

 at my command I must do as others had done before me; 

 namely, search for the chemical focus of any given com- 

 bination by taking a series of negatives until I had the 

 point at which the best picture could be obtained, I 

 came to the conclusion that the subject warranted inves- 

 tigation. I thereupon consulted with Mr. William Wales 

 and put the question to him as to whether he could not 

 make a lens specially corrected for photography ; namely, 

 one in which the chemical and visual foci would corre- 

 spond. In the meantime I turned my attention to the 

 further examination of the matter and put to myself this 

 question — supposing such a lens made, would its deflni- 

 tion be superior or inferior to those then in use ? And 

 to test this matter I decided to take such lenses as I had 

 or could procure and use only or almost only the chem- 

 ical rays that passed through them and then ascertain 

 if such a lens would be available for working purposes. 

 I find in my memorandum book for October, 1865, 

 the following record : "Having nearly two years since 

 made several experiments from which I deduced certain 

 inferences with reference to the character of the illumin- 

 ation used when resolving fine-lined objects, I made 

 some more today. My first experiments were these: I have 

 two objectives l-5ths, one a defines pretty well, h much 

 better, same power and angle. Why is this ? 



A. I put on a with B occular, Amphipleura, Cuba, a 

 fixed angle of oblique light. I did not see markings. If 

 I place a blue glass anywhere in the path of the ray I 

 see lines. 



B. I put on b in exactly the same circumstances and 



