Discovery of his Theory of Colours. 



123 



A/IOLET 



BLUE 



GREEN 



"A narrow 

 line of yel- 

 low." 



dark lines crossing the spectrum within the blue portion and 

 in its neighbourhood, in which the continuity of the light 

 seems to be interrupted. This distribution of the spectrum 

 Dr. Wollaston has found to be the same, whatever refracting 

 substance may have been employed for its formation ; and< he 

 attributes the difference which has sometimes been observed in 

 the proportions to accidental variations -p-^ g 



of the obliquity of the rays. The an- 

 gular extent of the spectrum formed by 

 a prism of crown glass is ^ of the 

 deviation of the red rays, by a prism 

 of flint glass j\" (fig. 3)*. 



Fig. 3. " The spectrum produced by 

 looking through a prism at a narrow 

 line of light. 



" In light produced by the combus- 

 tion of terrestrial substances the spec- 

 trum is still more interrupted : thus 

 the bluish light of the lower part of 

 the flame of a candle is separated by 

 refraction into five parcels of various colours ; the light of 

 burning spirits, which appears perfectly blue, is chiefly com- 

 posed of green and violet rays ; and the light of a candle 

 into which salt is thrown abounds with a pure 

 yellow inclining to green, but not separable by re- 

 fraction. The electric spark furnishes also a light 

 which is differently divided in different circum- 

 stances. 



" If the breadth of the aperture viewed through 

 a prism is somewhat increased, the space occupied 

 by each variety of light in the spectrum is aug- 

 mented in the same proportion, and each portion 

 encroaches on the neighbouring colours and is 

 mixed with them ; so that the red is succeeded by 

 orange, yellow, and yellowish green, and the blue 

 is mixed on the one side with the green, and on the 

 other with the violet ; and it is in this state that the V__^/ 



prismatic spectrum is commonly exhibited. 



Fig. 4. " The appearance of a circular aperture, moderately 

 large, viewed through a prism. 



" Sir Isaac Newton observed that the effect of white light 



* Figures 3 and 4 are copied of the exact size of those given t>y 

 Young in the plates appended to his ' Natural Philosophy.' The descrip- 

 tions of the figures are those given by Young. The colours in Young's 

 figures we have indicated in type. There are sis other figures illustrating 



K2 



