95 



and Lealand's new y^gth immersion objective, gave the following 

 results : — 



15th band ... 45 lines 18th band ... 54 lines 



16th „ ... 48 „ 19th „ ... 57 „ 



17th „ ... 51 „ 



In obtaining the above results I illuminated the microscope, as 

 in my former woi-k on the Nobert's plate, with a pencil of mono- 

 chromatic light obtained by reflecting the direct rays of the sun 

 from a heliostat upon a mirror, by which they were thrown 

 thx'ough a cell filled with a solution of the ammonia sulphate of 

 copper, upon the achromatic condenser. As an achromatic 

 coudenser I substituted for that belonging to the large Powell 

 and Lealand stand of the Museum a Jth of an inch objective of 

 148° angle of aperture, and used it without a diaphragm ; ob- 

 liquity of light was obtained by moving the centering screws of 

 the secondary stage. 



I also obtained satisfactory resolution of the nineteenth band, 

 with the same lens, by usiug for the illumination violet light, 

 obtained by throwing the violet end of the solar spectrum pro- 

 duced by a large prism upon the achromatic condenser used as 

 above, and, subsequently, by shifting the prism, got successful 

 resolution of the nineteenth band with blue, green, yellow, orange, 

 and red light. These results I had the pleasure of exhibiting to 

 Dr. Barnard and several others. 



As for other lenses, carefully tried on the same plate, I ob- 

 tained the following results : — 



The |-th of Wales and the J^th and ^i^th of Powell and 

 Lealand, all dry lenses, resolved the fifteenth band, but not the 

 sixteenth. 



An immersion ^'jth by Wales resolved the sixteenth band, but 

 failed to go farther. An immersion -^Qth. by Wales resolved the 

 seventeenth band, but failed to go farther. A Hartnack im- 

 mersion No. "11," belonging to President Barnard, also resolved 

 the seventeenth band, and f;dled to go farther. 



A Tolles' immersion Ith, just constructed for Dr. J. C. E-ives, 

 of this city, resolved the fourteenth band, but failed to show the 

 true lines on the fifteenth. This result with the Tolles' im- 

 mersion jth corresponds with the results very recently obtained 

 with a Tolles' immersion ^th, just received by my distinguished 

 friend, Mr. W. S. Sullivant, of Columbus, Ohio, who wrote me 

 May 25th of the present year: — "The immersion lens you in- 

 quired about, which Tolles sent me, was marked -^-th, but was 

 a strong Jth English standard. The utmost it could do was to 

 show true lines on the fourteenth band." 



These results confirm the opinion expressed in my former 

 article, that the lines claimed to have been seen, but not counted, 

 in the nineteenth by a Tolles' immersion ith were spurious lines, 

 an opinion to which still greater weight is added by the following 

 result : — A Tolles' immersion -j^'^-th of 175° angle of aperture was 

 received at the Museum, May 26th, from Mr. Charles Stodder, 



