NATURAL PHILOSOPHY. 145 



excited by a light enclosing rays iu their keeping. In this re- 

 spect, the white light can alone render visible the proioer color of 

 the whole body, because each one finds there rays capable of ex- 

 citing its vibrations. From thence if, in a studio receiving the 

 light from above, the red, orange, and yellow of a picture appear 

 obscure and black in tlie twilight, apparently the faint ci-epuscular 

 light of the raised part of the heavens does not send rays vibrating 

 in unison with these colors ; and if, on the contrary, the violet, blue, 

 and green are bright, it is because some rays in unison with 

 them have penetrated into the studio. Or, indeed, at sunset, the 

 elevated region of the heavens furnisli, according to Father 

 Secchi, a shortened atmospheric spectrum, destitute of red, 

 orange, and yellow, and enclosing only green, blue, and violet, 

 doubtless because the atmospheric prism refracts onlytovvards the 

 earth rays of the greatest refrangibility. From thence, as the 

 frangibility of the rays augment with the rapidity of their vibra- 

 tion, the absence of the red, orange, and yellow rays, in the zenith 

 crepuscular spectrum, is due to the insufficient vibratory rapidity 

 of these rays. On the contrary, the green, blue, and violet rays 

 owe their presence in the zenith crepuscular spectrum to their 

 greater refrangibility due to their greater vibratory rapidity. 



2. Cause of the Unequal Pliotographic Work of the Colors. — 

 The photographer transfers in black, the red, orange, and yellow 

 of a picture, and in milky-white the blue, indigo, and violet. It 

 is the same with the photographs after nature. Now these infideli- 

 ties result necessarily from the unequal photographic work of the 

 colors, for the pictures of monochromatic objects, of edifices, 

 statues, in gray, are perfectly faithful. In a monochromatic 

 painting, the luminous vibrations are all in unison, and have the 

 same amplitude under the same bright light. Now the efficient 

 work corresponding to the photographic impression represents, 

 for every part of the picture, an even amount of quick vibratory 

 forces absorbed. From thence the duration of the work will be 

 reciprocal to the sensibility of the photographic retina, and, with 

 equal sensibility, this duration will be reciprocal to the amplitude 

 of the vibrations, or to the intensity of the light ; in short, in equal 

 intensity with the light, the duration of the efficient work will be 

 reciprocal to the rapidity of the vibrations, and, in consequence, 

 to the refrangibility of the gray color, for it must be operated by 

 the same number of vibrations for each color, if all have the same 

 vibratory amplitude. Thence, the more rapid vibrations of the 

 more refrangible colors achieve their eflipacious work in a less 

 time than the slower vibration of tlie less refrangible colors ; and 

 they cannot impose, with impunity, the same duration to the pho- 

 tographic work of all the colors. For if this duration was regu- 

 lar upon the middle color of the spectrum, or upon the efficacious 

 work of the green, the vibrations of which are more rapid than 

 those of the yellow, orange, and red, these last colors, not having 

 been able to achieve their efficacious work, will be obscured with 

 black in their positive image ; on the contrary, the more rapid 

 vibrations of the blue, indigo, and violet, finish their efficacious 

 work before the green, their excessive vibrations will have swept 



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