58 SOLAR SPECTRUM. PART i. 



the air, and that they are more abundant in the lower 

 than in the higher strata of the atmosphere. Since 

 opalescent matter reflects the blue rays of light and 

 transmits the red, Professor Roscoe ascribes the blue 

 colour of the sky and the bright tints at sunrise and 

 sunset to the opalescent property of the air. 



The atmosphere is permeable to every kind of chemi- 

 cal rays, which is far from being the case with bodies 

 on earth, some of which though transparent to all the 

 visible rays, vary greatly in their transparency to the 

 chemical rays. 



The atoms and molecules of matter not only have 

 the power of turning the rays of the solar beam out of 

 their rectilinear path, but of changing their refran- 

 gibility. 



The myriads of ethereal waves or rays of light that 

 constitute the seven colours of the solar spectrum, 

 decrease in refrangibility and increase in rapidity of 

 vibration and length of wave from the extreme violet to 

 the end of the red ; each ray having its own rate of 

 vibration, its own length of wave, and its own colour. 

 From the middle of the yellow, which is the luminous 

 part of the spectrum, the chemical spectrum extends 

 invisibly, but with increasing refrangibility and in- 

 creasing velocity of vibration, to a point far beyond the 

 violet. On the contrary, the heat spectrum, which may 

 also be said, to begin in the yellow light, extends in- 

 visibly but with decreasing refrangibility, and decreas- 

 ing velocity of vibration to some distance beyond the 

 visible red. 



The rays of heat are absorbed by the humours of the 



i eye, but were they to reach the retina we should see 



I that they differ from one another as much as those of 



the luminous spectrum ; the chemical spectrum from its 



greater length is still more diversified. 



The whole of the solar spectrum, visible and invisible, 



