Polytechnic Association. 73 X 



once, the circumference and the diameter of the orbit. We know 

 the greatest angular distance between the stars ; we have then the 

 distance of the stars from the earth. 



The President — Mr. Lewis M. Rutherfurd, of this city, has been 

 quietly doing a work, at his own observatory, which will be consulted 

 50,000 years after we are forgotten, in photographing groups of stars, 

 by which their positions are exactly recorded, without the error of 

 personal equation. Mr. 1). Chapman, the photographer, who assists 

 him in his work, is present to-night, and we would like to hear from 

 him some account of that work. 



Mr. D. Chapman — The groups of stars are photographed twice a 

 year ; six months apart. The telescope takes directly a field of about 

 two degrees. There is a little distortion, but that can be ascertained 

 mathematically and applied as a correction. Some of the groups 

 contain as many as 125 stars, down to the ninth magnitude, taken 

 upon a plate five and one-half inches square. A star suspected of 

 proper motion is placed in the center of the plate. The plate is 

 exposed six minutes, and then the telescope is moved slightly and the 

 plate is exposed six minutes longer, duplicating all the stars upon the 

 same plate, so as to identify them from other spots upon the plate 

 and verify their positions. Then the clock-work is detached, and, 

 after a certain length of time, the clock is again attached and a sup- 

 plemental exposure is made, which furnishes us the base line, or zero 

 of position. With the micrometer, each star is determined in posi- 

 tion by its distance from the central star and by the angle of the line 

 from it to the central star with the east and west line. In order to 

 get rid of any constant error from the direction of the telescope, we 

 usually take the stars with the telescope first on one side of the pier,^ 

 and then, when the stars are in a different position, with the telescope 

 upon the other side of the pier. By repeating the groups six months 

 apart, we hope, by and by, to ascertain the amount of parallax of 

 some of them. We thus have an absolute map of these groups for 

 future use. On a cloudy day, a young lady sits down and measures 

 the positions of those stars. 



Dr. P. H. Yan der Weyde — Will you explain about making the 

 telescope achromatic for the actinic rays ? 



Mr. D. Chapman — The first object glass, that which Dr. Gould now 

 has at Cordova, was corrected for the actinic ray alone. The ordinary 

 photographic lens is a compromise between the visual and the actinic 

 rays. The thirteen inch objective we now have was first corrected 

 for the visual rays as perfectly as possible. Then a flint glass 



