PHOTOGRAPHIC PHOTOMETRY. 195 



then make use of the pliotographio photometer. Suppose we have a 

 holder fittiug a sensitive i)hite, and over the l-ntter a metallic shutter 

 pierced with holes of the size of the "stellar circles." Before this plate 

 let a second plate move by virtue of a spring, this second plate having 

 a triangular window. When the window, in consequence of the action 

 of the spring, passes before the holes of the shutter it will determine 

 for each of them a photographic action, which will be measured by the 

 width of the triangular window at that point and by the velocity of 

 the slide, which can be evaluated by a tuning fork. Thus a series of 

 circles of graduated intensity will be obtained comparable with the 

 "stellar circles." These circles are due to direct sunlight. But to ren- 

 der the results more strictly comparable it will be proper to lay the tele 

 scope objective down on the plate, so that the effects of absorption and 

 reflection may be the same as when the stars were photographed. 



By this method, the results of which were duly reported to the 

 Academy of Sciences,^ the radiating powers of several stars were inves- 

 tigated. In particular it was used to compare the light of our sun 

 with that of the brightest star in the heavens, the Dog Star, or Sirius. 

 It showed that that colossal orb had an intrinsic radiation in the i)ho- 

 tographic spectrum ten times that of the center of our planetary 

 system. 



But it is especially when we wish to measure the luminosity of special 

 parts of an object, and are not content with knowing the total radia 

 tion, that the advantages of the photographic method are manifested. 

 We have, for example, been able to evaluate the light of different j)arts 

 of the tail of comet 1881 b, and to give quite closely the law of the 

 decrease of that light as the distance from the nucleus increases. 



The arrangement employed was as follows: On tlie photograi)hic 

 plate, fitted into a holder, was placed a screen with an opening represent- 

 ing the comet's tail. Before the apparatus was a shutter in which a trian- 

 gle had been cut out, having its base rectilinear, but its sides curved. 

 When this triangular window moved in the direction of the line of its 

 base the times of exposure of the different parts depended on the forms 

 of the curved sides. The whole base moved along that point of the 

 opening in the screen that represented the nucleus, while the vertex of 

 the triangle passed over the i)art of the opening representing the 

 extreme and evanescent portions of the tail. Here the time of exposure 

 vanished. In intermediate parts, say, at distance x from the base, the 

 time of exposure was proportional to the distance, 2;/, from one side of 

 the triangle to the other along a line parallel to the base and to the motion. 

 A number of such shutters were constructed, the curves of the sides 

 being determined by one of the equations Ay = x"\ m having a constant 

 value for each shutter, but different values for different shutters. The 

 whole being exposed to uniform illumination and the shutter moved by 

 a spring an artificial figure of a comet was obtained with each shutter — 



» April 4, 1891. 



