222 HERSOHEL. 



the germs of a large number of beautiful experiments more fully devel- 

 oped in our own day. 



By successively placing the thermometer in all parts of the solar 

 spectrum, he determined the illuminating powers of the various prismatic 

 rays. The general result of these experiuients 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 brighest yellow and the palest green. The yellow and the green 

 possess this power equally. A like assimilation may be laid down be- 

 tween the blue and the red. Finally, the power of illumination in the 

 indigo rays, and above all in the violet, is very weak. 



The memoirs of Herschel on Newton's colored rings, though con- 

 taining a multitude of exact experiments, have not contributed much 

 to advance the theory of those curious i)henomena. I have learned, 

 from good authority, that he himself held the same opinion. He 

 said that it was the only occasion on which he had reason to regret 

 having, according to his constant custom, published his labors immedi- 

 ately as fast as they were performed. 



