ASTRONOMY. 117 



Stellar ])liotograpliy at Cordoba. — Dr. Gould, in a paper read at the 

 Butialo meeting of the American Association, lias described rhe photo- 

 graphs taken at Cordoba from 1872 to 1882. About seventy southern 

 clusters and more than a hundred double stars were repeatedly photo- 

 graphed. Some sixteen plates of the Pleiades and five of Praesepe 

 were obtained ; the total number of photographs being somewhat less 

 than thirteen hundred. Dr. Gould lays great stress on the necessity 

 of promptly converting the photographs into a permanent numerical 

 record : and considerable uneasiuess is aroused by the discovery that 

 the collodion or gelatine films are readily detached from the plates. 

 Some progress has already been made in the reductions, under Dr. 

 Gould's immediate supervision at Cambridge. 



Fritchard's '•'■Researches in stellar photography.''^ — In a paper with the 

 foregoing title, read at the meeting of the Royal Society, May 27, 1886, 

 Professor Pritchard gives an account of a number of photographs of 

 the Pleiades which he has submitted to a critical examination, with the 

 following objects in view : 



(1) To ascertain, by means of definite and accurate measurement, the 

 relation between the diameter of a star-disk impressed on a photo- 

 graphic plate with a given exposure, and its photometric magnitude; 

 a 8imi)le formula seems to connect the two. (2) To ascertain whether 

 the photographic plate remains an absolutely accurate picture of the 

 actual relative positions of the stars in the sky itself, and, moreover, 

 whether these are measurable with that extreme degree of jirecision 

 which is attainable with the best instrumental means. The satisfactory 

 accordances of measures of different X)lates have afforded a sufficient 

 answer to this inquiry. (3) The third subject of investigation was the 

 relation between the areas of the impressed star-disks and the time of 

 exposure of the plates. As far as at present appears, these areas vary 

 as the square root of the time, though the investigation is not to be 

 regarded as complete. Bond, in 1858, considered that the areas varied 

 directly as the time. 



In the course of his work Professor Pritchard noticed what appeared 

 to be a distortion of tlie photographic film on a small i^ortion of the 

 plate, and he detected a somewhat similar distortion upon one of eight 

 plates of 61 Cygni and neighboring stars. He has hopes that in the 

 course of a year the parallax of certain stars will be re-determined by 

 photography, even to a greater degree of accuracy than has hitherto 

 been achieved by direct instrumental application. 



Professor Harkuess has suggested that great increase in the accuracy 

 of transit observations of the sun would be gained by inserting a sensi- 

 tivephotographicplatejustbehindthewiresystemof theinstrument, and 

 making an instantaneous exposure at the time of the sun's transit. 

 This would avoid the disturbance of adjustments of the instrument 

 arising from the exposure to the sun for several minutes, which is nec- 

 essary in the present mode of observing. Stars would be observed and 



