480 HISTORY OF SCIENCE. 



In a note to a passage relating to the proportions of the colours in 

 the spectrum, he adds : 



" Although what I have above described comprises the whole of 

 the prismatic spectrum that can be rendered visible, there also passes 

 on each side of it other rays whereof the eye is not sensible. From 

 Dr. Herschel's experiments we learn that on one side there are invisible 

 rays occasioning heat that are less refrangible than red light, and on 

 the other I have myself observed, and the same remark has been made 

 by Mr. Ritter, that there are likewise invisible rays of another kind 

 that are more refracted than the violet. It is by their chemical effects 

 alone that the existence of these can be discovered, and by far the 

 most delicate test of their presence is the white muriate of silver. To 

 Scheele, among many valuable discoveries, we are indebted for having 

 first duly distinguished between radiant heat and light, and to him 

 also we owe the observation, that when muriate of silver is exposed to 

 the common prismatic spectrum, it is blackened more in the violet 

 than in any other kind of light. In repeating this experiment, I have 

 found that the blackness extended not only beyond the space occupied 

 by the violet, but to an equal degree and to about an equal distance 

 beyond the visible spectrum ; and that by narrowing the pencil of 

 light received on the prism, the discoloration may be made to fall 

 almost entirely beyond the violet. It would appear, therefore, that 

 this and other effects usually attributed to light are not owing to any 

 of the rays usually perceived, but to invisible rays that accompany 

 them ; and that if we include the two kinds that are invisible, we may 

 distinguish upon the whole six species cf rays into which a sunbeam 

 is divisible by refraction." 



These extracts show that Wollaston was the first who saw either the 

 dark lines in the solar spectrum, or the bright lines in the spectra 

 yielded by flames and by electric sparks. He did not, however, 

 pursue the subject, nor indeed does any notice appear to have been 

 taken of his observations. Some years afterwards, however, the dark 

 lines of the solar spectrum were observed under better conditions by 

 the German optician FRAUNHOFER, a native of Bavaria (1787 1826). 

 Fraunhofer appears to have been unacquainted with Wollaston's de- 

 scriptions of the dark and light lines of spectra ; and as he there- 

 fore not only discovered the dark solar lines for himself, but was the 

 first to study them carefully, they have been very properly called 

 Fraunhofer* s Lines. His method of observation was somewhat different 

 from that of Wollaston ; for, instead of looking through the prism with 

 the naked eye, he made use of a telescope, which, with the prism, was 

 placed at a distance of 24 feet from a very narrow slit. Not only was 

 his method of observation better than that of his predecessor, but his 

 appliances were of an excellence which could not previously have been 

 equalled. His prism, for example, was not only wrought with great 

 nicety, but was made of the famous glass specially prepared for optical 



