378 SCIENTIFIC RECORD FOR 1885. 



tember 26 served to determine the magnitudes. The plates showed 

 57 stars between the Gth and 11th magnitudes. It is certain that the 

 photographic determination of the relative positions of so many stars 

 represents a very considerable economy of time, and also guards against 

 personal errors. As for the errors of the photograph, they caii always 

 be detected by the simultaneous reprodu(;tion of a squared reticle. Five 

 successive exposures of l'^, 4*", 9"^, 10'", So'", respectively, were made for 

 the purpose of determining the magnitudes, the telescope being each 

 time moved in declination. Five series of images, each more numerous 

 than the last, were thus obtained. The i)hoto-chemical effect is known 

 to be proportional to the intensity of the light and to the time of the 

 exposure; in cases of equally distinct images, therefore, the brightness 

 of the stars must be inversely proportional to the time of exjjosure. 

 Dr. Lohse has proceeded upon this principle in determining the rela- 

 tive magnitudes of the stars in the cluster of x Persei, emjiloying Dr. 

 Vogel's actinometric data. (It seems that the photographic intensities 

 of two successive magnitudes are in the ratio of 1 : 3, while the ratio of 

 intensities as seen with the eye is that of 1 : 2'5.) Photographic magni- 

 tudes generally agree well enough with optical magnitudes; still there 

 are some exceptions : thus a red star called So by Dr. Yogel is marked 

 10-5 by Dr. Lohse. It would be interesting to apply the same process 

 to the actinometric study of variable stars. 



We must not neglect to speak here of the fact tUat MM. Henry have 

 obtained a very beautiful photograph of the star cluster of Perseus 

 at the Paris Observatory. They succeeded in locating 509 stars be- 

 tween the Gth and 13th magnitudes included within 1° of right ascen- 

 sion and l°-r) of declination." {Bull. Astron., March, 1885.) 



Stellar plwtogra'phy at Harvard College Observatory. — By the aid of the 

 Bache fund an important investigation has been undertaken in stellar 

 photography. It is found that stars as faint as the 6th magnitude, in 

 any part of the sky, can be readily photograi^hed, even without clock- 

 work; while near the pole, where the diurnal motion is slower, stars 

 as faint as the 14th magnitude may be photographed in like manner. 

 These results have much value as a means of determining the relative 

 positions and biightness of different stars. Charts of regions five de- 

 grees square can also be photographically prei)ared and enlarged by 

 photolithography to the scale of the maps drawn by Chacornac and 

 Peters. The spectra of stars have all been photographed with much 

 success. Stars as faint as the 8th magnitude give photographic spec- 

 tra in the paper prints, from which the lines can be distinctly seen. Mr. 

 W. H. Pickering has rendered important aid in this investigation. A 

 part of his researches upon the possibility of photographing tlie solar 

 corona, except during an eclipse, was conducted at the Observatory and 

 the results published in Science. 



The first stellar photographs ever taken were those of a Lyrse, by the 

 elder Bond, at the Harvard Observatory, in 1850. In 1857 his son car- 



