302 HEKSCHEL. 



ber of beautiful experiments since erected upon it in our 

 own day. 



By successively placing the same objects in all parts 

 of the solar spectrum Herschel determined the illumi- 

 nating powers of the various prismatic rays. The general 

 result of these experiments may be thus enunciated : 



The illuminating power of the red rays is not very 

 great ; that of the orange rays surpasses it, and is in its 

 turn surpassed by the power of the yellow rays. The 

 maximum power of illumination is found between the 

 brightest yellow and the palest green. The yellow and 

 the green possess this power equally. A like assimila- 

 tion may be laid down between the blue and the red. 

 Finally, the power of illumination in the indigo rays, and 

 above all in the violet, is very weak. 



Yet the memoirs of Herschel on Newton's coloured 

 rings, though containing a multitude of exact experi- 

 ments, have not much contributed to advance the theory 

 of those curious phenomena. I have learnt from good 

 authority, that the great astronomer held the same opin- 

 ion on this topic. He said that it was the only occasion 

 on which he had reason to regret having, according to 

 his constant method, published his labours immediately, 

 as fast as they were performed. 



