loo GREAT MEN OF SCIENCE 



the uprooting, that the period of 'enlightenment,' at any rate 

 the beginning of it, coincides with an almost century-long 

 pause in great scientific progress. This is the time from the 

 appearance of Newton's Principia (1686) to that of Watt on 

 the one hand, and Coulomb, Galvani, and Volta on the other, 

 who (all round about 1780 and 1790) created the foundations 

 of the new development we have mentioned. For this how- 

 ever, remarkably enough, the development of new means or 

 new tools of knowledge was in no way necessary, but merely 

 the re-adoption of modest and patient devotion to nature. 

 This is also true of the discoveries of Scheele, Priestley, and 

 Cavendish (in the years 1770 and 1780), who founded the 

 new chemistry. 



An important and outstanding part of Newton's investiga- 

 tions was his discoveries concerning the colours of light. 

 Here he appears entirely as an experimenter, as master of the 

 art of observation, who puts carefully considered questions 

 to nature by means of lenses and prisms. The only previous 

 example, although in a quite different direction, of experi- 

 mental activity so wide in extent, and so comprehensive and 

 illuminating in its effects, was the work of Guericke. A great 

 deal had been found out about light from the time of Euclid 

 to that of Kepler, and so on to Huygens; but the nature of 

 colour, the relation of coloured light to white light, the de- 

 velopment of colours in the rainbow, indeed everything in 

 which we have to do not with light alone, but with colour, 

 had still remained incomprehensible. The tendency was 

 to regard white light as the original thing given, as by the 

 sun, and as simple in its nature, and coloured light as com- 

 pounded of white light and something which colours it, 

 something coming from a coloured body and acting upon the 

 white light. Newton started from the colours which result 

 from white light without the assistance of coloured bodies, as 

 in the waterdrops of the rainbow, or in the colourless glass 

 of a prism used to refract light. By adding experiment to 



