196 PHOTOGRAPHIC PHOTOMETRY. 



that is, with each vahie of m. When m='2 the photographic action 

 would be inversely as the second power of tlie distance from the nucleus. 

 With m = 3, it would be inversely as the third power, etc. 



Now, the different pictures so obtained being compared with the 

 photograph of the real comet, it was found that the intensities could 

 best be matched in all parts by taking m between 4 and 0, so that the 

 result was that the intensity of the light in the comet's tail, in the 

 photographic part of the spectrum, varied inversely as the fourth to 

 sixth power of the distance from the nucleus.^ At such enormous rate 

 does the light diminish as we pass away from the nucleus. 



The method of "stellar circles" comes into play again when we wish 

 to reproduce the conditions under which any celestial photograph has 

 been taken, especially in the study of nebula?. A nebula is not an 

 object having a definite outline, like the sun, the moon, etc. Its image 

 is rather like a nimbus cloiul, its different parts differing greatly in 

 brightness. The consequence is that differences in the power of the 

 telescope, the time of exposure, the sensitiveness of the film, the trans- 

 parency of the atmosphere, etc., result in i)ictures so different that they 

 sometimes would not be supposed to possibly represent the same thing. 

 If, for instance, a nebula has brilliant parts scattered in it, a photo- 

 graph with short exposure will show these parts only as isolated from 

 one another, while with a long exposure the intervening ])laces will be 

 all filled up with little variation of intensity. In 1881 we obtained at 

 Meudon a series of photographs of the great nebula in Orion, which 

 show how surprisingly the aspect of that object may change with 

 greater or less exposure. 



Nevertheless, if we wish to hand down to posterity monuments which 

 shall allow changes in the nebula to be i)ut out of doubt, we must 

 contrive some way in which the photographs of future ages shall be 

 rendered comparable with those of today. Here the "stellar circles" 

 afiord valuable assistance. Suppose that on the plate which has just 

 received the photographic impression of the nebula we form stellar 

 circles of some five stars, well chosen, not variable, and situated in the 

 neighborhood. Then we shall obtain, after the development, along 

 with the inuige of the nebula those of the stellar circles of comparison. 

 The ratios of the times of exposure of the nebula and the circles will 

 be carefully noted. Later, then, w^hen the time comes to make a com- 

 l^arable photograph, we have only to ascertain the time of exposure 

 which, with a new telescope and a new photographic pre]>aration, is 

 required to produce stellar circles of the same diameter from the same 

 stars of equal intensity, and we find by the rule of three the time of 

 exj)osure requisite to obtain an equivalent photograph of the nebula. 



It is remarkable that we can, by such device, obtain, after any lapse 

 of time, no matter how different all the conditions may have become, a 

 photographic image altogether comparable with that of times gone by. 



1 Anauaire du Bureau des Longitudes for 1882. 



