THE SUN'S SPECTRUM 157 



still transmitted considerably more light than the red 

 glasses, they remedied the former inconvenience of an 

 irritation arising from heat. Repeating these trials 

 several times, I constantly found the same result." 

 How to see the sun distinctly without inconvenience 

 or danger from the heat continued to occupy his 

 thoughts for years. "I viewed the sun through 

 water," he wrote in 1801. " It keeps the heat off so 

 well, that we may look for any length of time, with- 

 out the least inconvenience." " Ink diluted with water 

 gave an image of the sun as white as snow ; and I saw 

 objects very distinctly, without darkening glasses." 



Herschel introduced his papers on the sun's light 

 and heat with a wise remark, which proved him to be 

 as good an observer in the world of mind as in that of 

 matter. " It is sometimes of great use in natural philo- 

 sophy to doubt of things that are commonly taken 

 for granted ; especially as the means of resolving any 

 doubt, when once it is entertained, are often within 

 our reach. ... It will therefore not be amiss to notice 

 what gave rise to a surmise, that the power of heating 

 and illuminating objects might not be equally distri- 

 buted among the variously coloured rays." The ex- 

 periments, which he then made on the light and heat 

 given out by each colour of the spectrum, were admir- 

 ably imagined and beautifully carried out. He was really 

 engaged on a continuation of Newton's experiments on 

 sunbeams, but the field of research was new and untrod. 

 Gradually the questions to which he sought answers 

 began to take shape more distinctly in his mind. 

 When a prism intercepts a beam of sunlight, let into 

 a darkened room through a hole in the window shutter, 

 and the band of coloured light, five times as long as it 



