192 PHOTOGRAPHIC PHOTOMETRY. 



So, also, tbe study of the planets would be advanced by photometric 

 observations which should afford a knowledge of the relations of their 

 surfjices and atmospheres to light. 



A special importance attaches to photographic photometry in its 

 ai)plication to the tixed stars. The quantity of light a star sends us 

 depends chiefly on its distance and its radiating power, so that either 

 of these two factors being known the apparent light will determine the 

 other. Thus, if the parallax of a star and the amount of light which it 

 sends to us are known, we can deduce the intrinsic amount of its radia- 

 tion—that is to say, its rank among the hosts of suns with which the 

 spaces of heaven are sown. Add to this knowledge that of the qualities 

 of its light, and you will have a total from which the magnitude, the 

 constitution, and the activity of the luminary in question may be almost 

 completely defined. 



Similar remarks apply to comets and uebuhe. Thus, celestial pho- 

 tometry constitutes one of the most important methods of astronomy; 

 and it is very interesting to see what aid it can derive from photog- 

 raphy. For this purpose I have employed a method which I proceed 

 to describe.' We know that in the common photometric method the 

 ratio of emission of the two sources of light is obtained by removing 

 the brighter of them until they each illuminate the shadow due to the 

 other equally. This is one of numerous methods not hitherto used in 

 photographic photometry. In a photograph, in order to judge of the 

 brightness of a luminous or illuminated body, we have nothing but 

 its i)hotographic action, or the greater or less oi^acity of the metallic 

 deposit which it occasions on the sensitive film. Now, the blackness 

 of this deposit is very far from being proportional to the time of action 

 of the light. Could we see the deposit increase, and could we measure 

 its amount while the action of a steady light was going on, we should 

 find that it increased rapidly at first, and afterwards slower and slower, 

 until it became practically stationary. Thus, we can not take as meas- 

 ure of the intensity of a source of light the degree of opacity of the 

 metallic deposit which it occasions, since that opacity is not propor- 

 tional to its action during a determinate time. But if, in place of con- 

 sidering the different degrees of opacity in relation to equalitj^ of the 

 time of action, we consider on the contrary the variable time necessary 

 to obtain a deposit of one fixed oi^acity, we shall have a sure basis for 

 the comparisons, and this, in fact, api)ears from experiment. 



The fact is, that to produce in a sensitive film a metallic deposit of 

 given blackness a certain sum of radiant action is needed, and whether 

 this sum is brought into existence in a longer or shorter time makes no 

 difference. Hence it follows that the energy of a source is increased 

 by the time necessary to effect the given degree of blackness — that is to 

 say, according to this principle, two sources are to one another inversely 



' Comptes rendus de I'Acaddmie des Sciences, XCII, 1881, April 4. Annuaire du 

 Bureau des Longitudes for 1882. 



