THE DIFFRACTION SPECTRUM. [MEMOIR VI. 



measures are obtained from a flat surface, and not upon 

 a circular arc. 



To the diffraction spectrum thns formed I exposed for 

 half an hour a daguerreotype plate, rendered sensitive 

 by iodine and then by bromine. It resulted that the 

 bromide of silver is decomposed at a maximum by a 

 wave which is 0.00001538 of a Paris inch in length. The 

 action does not extend equally, as we might have sup- 

 posed, towards the more and less refrangible regions. 



I exposed a silver plate which had been prepared by 

 iodine, bromine, arid chloride of iodine successively. The 

 point of maximum fell as before at 0.00001538. The 

 time of exposure one hour. The decomposition com- 

 menced by a wave in the green space, the length of 

 which was 0.00002007, and was terminated by one in 

 the violet, the length of which was 0.00001257. The 

 point of maximum action, therefore, inclined to the vio- 

 let, and was not midway between the extremities of the 

 photograph. The absolute length of the stain depends, 

 however, on the time of exposure. 



I need not multiply these results. It is sufficient to 

 add that in several trials I obtained in these delicate ex- 

 periments photographs of the diffraction spectrum on 

 different surfaces in great perfection. The fixed lines 

 which are crowded close together were beautifully dis- 

 tinct. 



I would suggest, therefore, that when we wish to in- 

 dicate spectrum regions with precision, we should use 

 wave-lengths. By doing this we shall connect the vari- 

 ous actinic phenomena with a great many of the numer- 

 ical results of optics, and have fixed points of comparison. 



UNIVERSITY OF NEW YORK, June, 1845. 



