

MEMOIII VII.] STUDIES IX THE DIFFRACTION SPECTRUM. 



mon pliotograpbic camera, which contains the ground 

 Lrlass and shields for sensitive preparations." 



In the publication above referred to I gave engravings 

 of the results thus obtained; the fixed lines were marked 

 by their wave-lengths. The photographs were very clear 

 and beautiful ; they bore magnifying six or eight times 

 without injury to their sharpness. 



I may here be permitted to add that it was on the 

 publication of these researches in 1844 that I first made 

 the suggestion to describe spectrum effects by wave- 

 lengths, or what, perhaps, is still better, by ether vibra- 

 tions a method now generally adopted. I may give 

 the following extracts: 



" In the earlier discussions of the chemical effects of 

 light, the different regions of the spectrum were marked 

 out by the designations of the different colored rays, and 

 effects were described as taking place in the red or yel- 

 low or violet regions respectively. An improved plan 

 was proposed by Herschel, and followed by him in his 

 various writings. It consists essentially in dividing the 

 space which exists between the red and yellow ray as 

 insulated by cobalt blue glass into 13.30 parts, taking 

 the centre of the yellow ray as the zero point, and 

 continuing the divisions equally into the more and less 

 refrangible regions. 



u Over these methods the use of the fixed lines pos- 

 sesses very great advantages, inasmuch as we make refer- 

 ence to actually visible points existing in the spectrum. 



"It has been stated that the deviations of the different 

 fixed lines in the diffraction spectrum are proportional 

 to the lengths of the undulations which they respective- 

 ly represent. By designating the different points of the 

 spectrum by their wave-lengths, the subdivision may be 

 carried to any degree of minuteness, the measures of one 

 author will compare with those of another, and the dif- 



