104 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Tlie gciieiiil equation connecting the conjugate foci of a simple lens is 



1 1 _1 



-»+ V- f 



where n, r, anil /are to be measured from the lens, and taken as positive when in the direction of 

 ]>n)i)agation of the liglit. Tlien, for this case 



_1 1_ 1 



u A 

 V ^ a 



L = F — « + V 

 Thus, in tlie instruiiitMit wliich Mr. Brown used 



A = 6o 

 and I wished to make A = 2°; moreover, 



F = 113 inches ; — /= 2 inches. 



These constants substituted gave 



u = — 4 inches; r = — L\ inches; L=14[\ inches, 



and the brightness of the spectrum, with a slit of the same angular width, was three times what it 

 would ha\ c been without the additional lens. It is evident that the apjiaratus admits of large 

 variation; for exami)le, had 1 chosen 



then I should have had 



V = — inches; r = — li inches; L=1G:J inches, 



and the brightness would have been increased four times. I should have ]>referred the latter pro- 

 portions had the tubing at our disposal admitted. 



4. 6ratm<i spectroscope. — Tliis was attached to my own equatorial, of 4 inches aperture, aiul 

 contained a grating by Kowland of about 15,000 lines and an eflective aperture of nearly 1 inch. 

 The apparatus was used by Mr. C. H. Rockwell. There was also a Hint-glass prism which could 

 replace the grating, but it was not used during totality. 



5. Prism spectroscope. — This was used in conjunction with Mr. Rockwell's fijj-inch equatorial 

 by the writer. The objective of the equatorial has a focal length of 91 inches. The objectives of the 

 collimator and viewing telescope have focal lengths of 12.8 inches and apertures of 1.6 inches, the 

 available aperture of the single 60° tlint-glass prism being also 1.6 inches. From the above dimen- 

 sions it is evident that the effective aperture for central pencils was but 0.9 inch ; but as the slit 

 was made nearly an inch in length, and used radially, this was as much as could be employed under 

 the condition that the whole pencil from nearly ev^ery portion of the slit should be transmitted. 

 Tlie magnifying power was 8, this yielding the maximum brilliancy under the assumption that the 

 diameter of the pupil was ^ of an inch. Of course it was better to have the ocular pencils small 

 rather than too great. The eye-piece had an angular field of 52°, which admitted the whole spec- 

 trum within the field, while at the same timet he power was sufficient to show the D lines obviously 

 separated. 



