208 PROCEEDINGS OF THE AMERICAN ACADEMY 



a Browning nine-prism direct-vision spectroscope (without slit) placed 

 inside tlie locus of the 28-inch mirror, the sensitive plate heing at the 

 focus. 



The 12-inch refractor was received at this time, and a good deal of 

 photographic work was done with it to determine its actinic focus and 

 other constants, but no photographs of stellar spectra were made with 

 it this season. Its best actinic focus was found to be -fj^^ inch outside 

 of the focus for G. 



In July. 1876, several photogi-aphs of the si)ectrnm of Vega were 

 taken with an apparatus which Dr. Draper called the '-spectrograph." 

 It consisted of a box about three feet long, which screwed into the 

 tailpiece of the reflector in place of the eyepiece. It consisted of the 

 following parts: first, a slit; close behind this the brass tube of a 

 Browning direct-vision spectroscope containing either 3, (>, or '.) prisms 

 (variable at pleasure) ; next to this, and 14 inches from the slit, a 7-iiich 

 Voigtlander portrait-lens and camera. The results were not materially 

 different from those obtained by the earlier methods, and the apparatus 

 was so awkward that it was soon abandoned. At this time was intro- 

 duced the plan of setting the slit in the direction of the right-ascen- 

 sional motion, so that any slight irregularities of the driving-clock 

 would only widen the spectrum a little, instead of removing the star's 

 image from the slit. The use of a cylindrical lens to broaden the 

 spectrum was not found to be of any particular advantage. 



During September and early in October, 1876, experiments were 

 tried by putting the Browning direct-vision prism, without slit or lens, 

 inside the focus of the 12-inch refractor. A cylindrical lens of 14-inch 

 focus could be placed either between the prism and the object-glass, or 

 between the prism and the sensitive plate. The difficulty produced by 

 the fact that the focus of the object-glass varies for diflisrent rays, was 

 partly overcome by tilting the sensitive plate. 



On October 9 the Huggins star-spectroscope seems first to have 

 been brought into use, one prism only being employed. It was at- 

 tached to the refractor, and at first used with wide open slit. It was 

 found difficult but very important to adjust the collimator accurately 

 in line with the optical axis of the large telescope. 



On October 12 another stellar spectroscope was arranged, consisting, 

 first, of a slit with an open space between it and the end of the tele- 

 scope, so that one could see whether the star remained centred on the 

 slit; then the nine-jirisin Browning direct-vision combination; then 

 two opera-glass lenses, and behind this the plate-holder. This was 

 attached to the reflector, and several stellar spectra were photographed 



