26 A. Hall on the application of Photography 
The first photographs of double stars, made for the purpose 
of determining by micrometric measurement their relative an- 
gle of position and distance, were those of Mizar and its com- ~ 
panion, made at the Harvard College Observatory in April and — 
May, 1857, by Messrs. Whipple and Black of Boston. Professor — 
. P. Bond has given in the Astronomische Nachrichten, No. 
1105, the results of his measurements of the thirteen photo- — 
graphic images. The zero of the angle of position was foun 
y moving the telescope in right ascension after an impression — 
had been taken and taking a second one on the same plate. 
This process repeated gave a series of photographic images on — 
the same plate and the right line passing through the series — 
gave the son of the daily motion of the heavens. The 
robable error of a single measurement of the photographic 
distance of the images was found to be + 0°12, or somewhat — 
smaller than that of a direct measurement with the common 
filar micrometer. During the summer of 1857 many more 
photographs of this double star were taken, and in the journal — 
above mentioned, No. 1129, Prof. Bond has given an elaborate — 
account of the measurement of sixty-two photographic images. 
For this measurement of these images a new and improved micro- 
meter was employed, consisting of an achromatic microscope wit. 
a magnifying power of 2200. Under this microscope the star — 
images appeared as aggregations of minute drops of matter and 
looked like the result of a great number of vibrating and — 
momentary impressions. They were generally symmetrical with — 
a gradual condensation toward the center of the image, and 
the bisection of an image with the wire of the micrometer could _ 
be done with great exactness. The probable error of the center 
of an image was found to be 0-051, and hence that of the 
distance of two such centers to be +0°’072. Adopting the 
estimate of Struve, +0°”127, as the probable error of a single 
measurement of a double star of this class with a filar microme- 
ter, the measurement of the photographic images would have a 
relative value three times greater, or = (9:123). The accuracy 
of the measurement is better shown by the numerical results 
obtained by Prof. Bond, which are as follows: 
Mean Exposure. Distance. No, of images. 
13° 14°31 + 0°034 7 
16 14°19 + 0°035 7 
18 14 *18 + 0°033 8 
24 14 °23 + 0°035 8 
25 14°15 + 0°034 7 
30 14 °28 + 0°034 2 
33 14°19 + 0°033 8 
36 14 -20 + 0°032 10 
24-5 14-21 + 0-013 2 
