90 Prof. Draper on the Existence and Mechanism 



stance could the extra-violet protecting action he traced, though 

 the analogous action of the red, orange, yellow, green and blue, 

 was perfectly given. Supposing, therefore, that the difference 

 must be due either to impurities in the iodine or to differences 

 in the method of conducting the experiment, I tried it again 

 and again in every possible way. To mi/ surprise I soon found 

 that the negative ejfcct tvas gradually disappearing ; and on 

 Sept. 29 it could no longer be traced, except at the highest 

 part corresponding to the yellow and green rays. In De- 

 cember it had become still more imperfect, but on the 19th of 

 the following March the red and orange rays had recovered 

 their original protective power. It seemed, therefore, that in 

 the early part of the year a protective action had made its 

 appearance in the red ray, and about July extended over all 

 the less refrangible regions, and as the year went on it had 

 retreated upwards. 



Are there then periodic changes in the nature of the sun's 

 light? The absorptive action of the earth's atmosphere is out 

 of the question : if that were the cause, the character of these 

 spectrum impressions should vary with the hour of the day. 

 Or is it not more probable that these singular phenomena 

 rather depend on incidental changes in the experiment, such 

 as external temperature, variations of moisture, the colour of 

 the sky, &c. ? 



Under proper circumstances there is no difficulty in exhi- 

 biting the power which the less refrangible rays exert in ar- 

 resting the action of the daylight: under such circumstances 

 a Daguerreotype impression of the sun's spectrum yields all 

 three of the varieties of surface before alluded to. The plate 

 in the less refrangible and extreme violet region is unaffected ; 

 a narrow space of white separates these unaffected portions 

 from the indigo and violet spaces, which are in a highly so- 

 larized state. 



But a totally different result is obtained when the daylight 

 is not allowed to fall on the plate, either before or during its 

 exposure to the spectrum. Under these circumstances the 

 rays which would otherwise protect now act on the plate and 

 slowly whiten it. A Daguerreotype spectrum formed in dark- 

 ness and without previous exposure to the light, exhibits a 

 white stain over all the less refrangible regions, and bears a 

 marked contrast to one formed under the simultaneous action 

 of a weak daylight. For brevity I will call the former the 

 spectrum of darkness, and the latter the spectrum of daylight. 

 The following are some additional observations. 



In the spectrum of darkness there is in the white stain a 



