yellow Bays un Daguerreotype Plates. 201 



this peculiarity, on my return to New York the followins; 

 August I made many attempts to obtain similar specimens, 

 but ill no instance could the extra-violet protecting action be 

 traced, though tiie analogous action of the red, orange, vellow, 

 green and blue, was perlectly given. Supposing, therefore, 

 that the difference must be due either to impurities in the 

 iodine or to differences in the method of conducting the ex- 

 periment, I tried it again and again in every possible way. 

 To my surprise I soon found that the negative effect was gra- 

 dually disappearing; and on Sept. 29 it co(dd no longer be 

 traced, except at the highest part corresponding to the yellow 

 and green rays. In December it had become still more im- 

 perfect, but on the 1 9th 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 protec- 

 tive action had made its appearance in the red ray, and about 

 July extendeci 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?" &c. 



From these experiments of Dr. Draper it would appear 

 that, according to the months of the year in which we operate, 

 the red and yellow rays either do or do not exercise a destruc- 

 tive action. It must be remarked that Dr. Draper mention's 

 only iodised plates, that he always speaks of iodine alone, and 

 that he never alludes to bromine or chlorine, which were 

 hardly in use at the time of his experiments. 



Dr. Draper adds (page 91), "I further found that when 

 different rays are brought to act upon each other, the result 

 does not alone depend upon their intrinsic differences, but 

 also on their rel uive intensities. Thus the green and lower 

 half of the blue rays, when of a certain intensit}', protect the 

 plate from the action of the daylight ; but if of a less intensity, 

 they aid the daylight. 



"The red and orange rays, when of a certain intensity, in- 

 crease the action of daylight on the plate; but if of a less in- 

 tensity, they restrain it," 



It would result Irom this last observation of Dr. Draper, 

 that when the red and orange rays are not endowed with the 

 destructive action, they, on the contrary, have the property 

 of continuing or assisting the action of daylight. 



I^ it not then possible, that, like Di'. Draper, I may have 

 made my first experiments on the iodized |)late during the 

 peiioil when the red and yellow rays were endowed vviih their 

 destructive action, and tluit Messrs. Beccjuerel and Gaudin 

 may have made theirs when these rays had lost their destruc- 



