10 THE CHEMISTRY OF THE SUN. [CHAP. 



This experiment will enable us to see the importance of 

 a precaution which we owe to Newton. In his very first 

 experiments he found that when we wish to get the best 

 possible effect out of a prism, we must so arrange it that the 

 particular ray which we wish to observe, whether the yellow, 

 the blue, the green, or any other, leaves that prism at exactly 

 the same angle as the incident compound ray falls on it. This 

 angle is termed the angle of minimum deviation. 



So much, then, for what we have learnt from Newton 

 touching sunlight. He was far from grappling with the grand 

 problem of solar chemistry ; but he saw at once that it was 

 impossible to imagine the sun to be a cool body, or to assume 

 for it a different origin from our own planet. In his Optics he 

 asks, " Are not the sun and stars great earths vehemently hot ? " 



When Newton made his classical experiments, lie used, as 

 we have already stated, a beam of light coming through a circular 

 hole in a shutter ; but he was soon able to prove to himself 

 that the circular aperture was not the best thing he could use, 

 because in the spectrum he had a circle of colour representing 

 every ray into which the light could be broken up, and all 

 these circles overlapped and produced a mixed and very impure 

 spectrum. But although he was conscious of the defects of 

 his method, he did not take the best steps to remedy them. 

 We must never forget that in his time the art of making 

 glass had made but a small advance. It was not until the 



O 



lapse of 140 years that another step forward was made by 

 Dr. Wollaston, who first employed a very narrow linear slit, 

 an arrangement which gives a very pure spectrum, and is, 

 therefore, adopted in all modern instruments. 



In consequence of this improvement, Wollaston was enabled 

 to make a discovery of the. highest importance, the first real 

 step, in fact, taken in Solar Chemistry, 



No sooner had he introduced the fine slit for the examina- 

 tion of the solar spectrum than he observed that the spectrum 



