POWER, ANTAGONIST FORCES. 177 



taken place.* In tropical climes, where a brilliant sun 

 is giving the utmost degree of illumination to all sur- 

 rounding objects, all photographic preparations are 

 acted upon relatively more slowly than in the climate of 

 England, where the light is less intense. As a remark- 

 able instance of this fact, a circumstance may be 

 mentioned, which is curiously illustrative of the power 

 of light to interfere with actinism : 



A gentleman, well acquainted with the Daguerreo- 

 type process, obtained in the city of Mexico all the 

 necessary apparatus and chemicals, expecting, under the 

 bright light and cloudless skies of that climate, to pro- 

 duce pictures of superior excellence. Failure upon 

 failure was the result; and although every care was 

 used, and every precaution adopted, it was not until the 

 rainy season set in that he could secure a good Daguer- 

 reotype of any of the buildings of that southern city. 



The first attempts, which were made at the instigation 

 of M. Arago, by order of the French Government, to 

 copy the Egyptian tombs and temples, and the remains 

 of the Aztecs in Central America, were failures. 

 Although the photographers employed succeeded to 



* " Having noticed, one densely foggy day, that the disc of the 

 sun was of a deep red colour, I directed my apparatus towards it. 

 After ten seconds of exposure, I put the" prepared plate in the 

 mercury box, and I obtained a round image perfectly black; the 

 sun had produced no photogenic effect. In another experiment, 

 I left the plate operating for twenty minutes ; the sun had passed 

 over a certain space of the plate, and. there resulted an image seven 

 or eight times the sun's diameter in length ; it was black through- 

 out, so that it was evident, wherever the red disc of the sun had 

 passed, not only was there a want of photogenic action, 'but the 

 red rays had destroyed the effect produced previous to the sun's 

 passage. I repeated these experiments during several days 

 successively, operating with a sun of different tints of red and 

 yellow. These different tints produced nearly the same effect ; 

 wherever the sun had passed, there existed a black band." Mr. 

 Claudet, On different properties of Solar Radiation, modified by 

 coloured glass media, fyc. : -Phil. Trans. 1847. Part 2. 



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