PHOTOGRAPHIC PHOTOMETRY. ^ 



By M. J. Janssen. 



A dozen years ago I undertook to lay the foundations of a science 

 of photographic photometry, with special reference to its application 

 to astronomical physics. I applied the fundamental principle of my 

 researches — a principle to be restated below — to the measurement of 

 the comparative intensities of solar and lunar light, to that of the earth 

 light on the moon, to the radiating activity of the sun as compared 

 with several stars, notably Sirius, and finally to the law of decrement 

 of the light in passing away from the head along the tail of the comet 

 C of 1881, which had been photographed at Meudon, the first among 

 such bodies. 



Perhaps these researches were not as energetically followed up by 

 me as they ought to have been, for their astronomical importance is 

 great, and they might have furnished valuable terms of comparison for 

 the future. But I have had the satisfaction of seeing the princiijle 

 adopted by the International Congress of Photometry for the measures 

 of photometric order for which they laid down the rules. In order 

 to renew attention to the subject, I here reproduce a resume of my 

 researches, wishing that it may become the starting point of new inves- 

 tigations. 



Photometry has to determine the ratio of intensities of two lights. 

 For example, to determine the ratio of luminosity of a candle and a 

 Carcel lamp is one jiroblem of photometry, and to evaluate in Carcel 

 lamps the power of an electric source of luminous radiation is another. 



We often have to make comparisons of this sort in the heavens. They 

 are always highly interesting and sometimes lead to beautiful discov- 

 eries. Thus, we know that the moon is an opaque ball not self-luminous, 

 but shining because lit up by the sun's rays. Now, we may ask what 

 proportion of the incident light it remits, in what measure the surface 

 shows specular reflection, and what is the relative amount of light of 

 the full moon as compared with that of the luminary from which its 

 rays are borrowed. Independent of the curiosity of such data in them- 

 selves, it is clear that if the study of them is carried deep enough it 

 may lead to important conclusions relative to the nature of materials at 

 the surface of the moon. 



1 Translated from Annuaire <Ju Bureau dgg Longitudes, Paris, 1895. 



191 



