302 HERSCHEL. 
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. 
