92 president's address — section a. 



the photographs of 1890 are simply masses of stars, that a group 

 that Herschel with, his great telescope estimated to contain 200 

 stars, on the photograph contains 14550, and that a well-defined 

 portion in Sagittarius, which in the 1890 plates contained eighty 

 stars, is now found to contain 1166, or fourteen times as many. 



Professor Kapteyn, from his study of photographs taken at the 

 Cape of Good Hope, was able to announce in March, 1893, that 

 stars near the Milky Way and in it are photographically brighter 

 than stars of the same visual magnitude which are at a distance 

 from the Milky Way, and the diiference is in proportion to the 

 distance. 



The photo- spectrograpliic method of measuring star motions 

 has already been referred to, but the results have recently, in tne 

 hand of Dr. Kempf, given a new and quite independent determina- 

 tion of the rate and direction of the sun's motion in sjiace. 

 Dr. Vogel thought that fifty-one stars were not enough to give the 

 result desired, but as the present apparatus is not powerful 

 enough to determine the motion of any more stars the comjiuta- 

 tion was made, with the result that " the apex of the sun's way " is 

 situated in R.A. 206° and north declination 46°, in the constellation 

 Bootes, and that its motion in that direction is at the rate of 

 eleven and a half miles per second. Many previous attempts have 

 been made to locate the '• apex of the sun's way," and they 

 jjlaced it in about R.A. 267° and north declination 31°. This 

 older method affords no means of determining the rate of the 

 sun's motion, unless an assumption was made as to the distances 

 of certain stars, and this made the velocity sixteen miles per 

 second, which does not differ very much from eleven and a half — 

 the value determined from phonographs. 



As an index of the great accuracy attained at Potsdam in 

 determining motion in the line of sight, it may be mentioned 

 that six photographs of the spectrum Arcturus were taken, from 

 which its motion in the line of sight was determined, and 

 Professor Keeler, using the great JAck telescope on three nights, 

 determined the same quantity by eye measurements, and the two 

 values agree within the tenth of a mile per second. 



Professor Keeler, using the great Lick telescope, 36in. in 

 aperture, has determined the motion of several nebidse in the line 

 of sight, and finds values ranging from two to twenty-seven miles 

 per second, and in one case forty miles per second. 



In this brief outline of what photography has done, and is 

 doing, much has been omitted for want of space, and in many 

 places the bare facts are given in order of time simply to recall 

 important steps in the progress to your memories. Kven in its 

 infancy photography was received kindly by astronomers, and 

 although much was expected from it nobody dreamt what it would 

 be to-day. Sir George Airy, as we have seen, was very much 

 impressed with what he saw, and he felt that a new power in 



