2-34 PROCEEDINGS OF THE AMERICAN ACADEMY 



to him, even more absorbingly interesting investigations, and urgent 

 business occupations. 



Tlie observations were made in his private observatory at Hastings- 

 on-the-Hudson, Lat. 4U° 59' 25", Long. 73° 52' 25". Elevation 

 above sea, 220 feet. 



The pictures of the Observatory and of the Great Equatorial render 

 unnecessary any detailed description of the mounting and general 

 arrangement of the instruments. 



The difficulties of the research proved to be very great. At first 

 the limitations imposed upon the time of exposure by the use of the 

 wet process made it almost impossible to get impressions of suilicic^nt 

 strength. This difRculty. however, is now a thing of tlie past, having 

 vanished with the introduction of the modern dry-plate processes. 

 Another difficulty, however, which increases with the time of exposure, 

 is that of securing a sufficiently accurate movement of the driving- 

 clock. Dr. Draper was obliged to construct no less than seven before 

 he succeeded in getting one that was perfect. Other difficulties which 

 were more or less completely overcome relate to the firm and rigid 

 connection of the parts of the spectroscope with each other, and with 

 the sensitive plate ; to the effect of temperature upon this connection, 

 and upon the dispersive power of the prisms employed ; and to the 

 method of obtaining a satisfactory reference spectrum for comparison 

 with that of the star under examination. Of course, also, every one 

 knows that operations of this kind are much more sensitive than 

 visual observations to atmospheric conditions. A slight haze, which is 

 rathei" an advantage than otherwise to ordinary work, cuts off the 

 actinic rays to such an extent as to increase the needed time of ex- 

 posure many fold. On some evenings, apparently good, it will take 

 30 minutes or an hour to obtain a picture as intense as could be ob- 

 tained on others in 5 or 10 minutes. 



Another serious practical difficulty should also be mentioned, — tlie 

 fact that Dr. Draper's residence was distant more than two miles from 

 his observatory ; and this of course involved many absolute disadvan- 

 tages and some loss of opportunities, as well as much inconvenience. 



It is not necessary to give here any fidl description of the telescopes 

 employed. It is enough to say that the great reflector constructed by 

 Dr. Draper himself has a mirror of silvered glass 28 inches in aper- 

 ture, with a focal length of 148 inches. It was generally fined up in 

 the Cassegrainian form, the small convex mirror, also of silvered glass, 

 having a diameter of 8 inches and a ncixative focal leniith of 29 inches. 

 It was placed 33 inches inside the principal focus of the great mirror. 



