I 



PHOTOGRAPHIC PHOTOMETRY. 193 



as the times they occupy in producing n given shade upon tlie same 

 sensitive film, or in other words, to produce equal photographic effects. 



This is the principle I have adopted in my investigations into photo- 

 graphic photometry. I have assured myself experimentally of the 

 legitimacy of this principle by ascertaining the relative amounts of 

 tune necessary to obtain tints of the same opacity on one sensitive film 

 placed at increasing distances of the source of liglit. It is found that 

 these times increase as the squares of the distances. Thus, for dis- 

 tances 1, 2, 3, 4 - - - 8, etc., the times are in the ratio of 1, 4, 9, 

 16 - - - 64, etc. It is necessary to Avork with one and the same 

 film, to avoid all stray light, to develop the plates in one bath, and, 

 in a word, to use all precautions possible in order to establish this law. 

 We may also mention that to obtain the best results possible it is need- 

 ful to take care to choose the opacity which corresponds to the most 

 rapid variation which happens toward the beginning of the action. 



Let us see how this method can be applied to the photometric study 

 of the heavenly bodies. 



For a long time astronomers and physicists have sought to determine 

 the luminous intensity of the moon relatively to that of the sun. 

 Bonguer appears to be the one who came nearest to the truth. Employ- 

 ing a candle as middle term of the comparison, he found the light of 

 the full moon at its mean distance to be about aoirooo of that of the 

 sun. The employment of a candle is open to criticism, because its light 

 is of a hue much warmer than that of the sun and still more so than 

 that of the moon — a circumstance which must have introduced peculiar 

 difficulties in making the i)hotometric matchings of shades. Bonguer 

 would have much improved his method had he subjected the caudle- 

 light to the sifting action of a suitable l)lue glass, so as to brings its 

 color nearer to that of the lunar rays. 



However that may be, it is remarkable that that old determination 

 agrees closely with the photographic measures which I obtained in 

 taking series of solar and lunar images and comparing the times of 

 exposure to which correspond photographs of the same intensity.' It 

 is to be farther noticed that Bouguer's determination refers to the 

 integral of visible rays, while the photographs are caused by more 

 refrangible rays. 



One could only infer that the lunar rays are highly photographic and 

 have great intensity in the blue and violet. The hue of moonlight 

 might have enabled us to foresee that result. 



The comparison of the light of the sun and moon speaks volumes in 

 regard to the admirable elasticity of our visual organ. When a country 

 is lit up by the full moon it only receives gooVoo of the light of noon- 



' Instead of taking photographs of the luminaries themselves, we can also, as I 

 did at the time, avail ourselves of bands of successive intensity obtained by sun. 

 light and moonlight. Thus, for the moon, we exposed to its light a holder whose 

 curtain uncovered successively and at etjual intervals of time the different parts of 

 a sensitive plate, giving, after (levelopuicnt, a series of bauds of increasing opacity, 

 ■which were compared with a plate exposed to sunlight for ^i/utr second. 



SM 94 13 



