43S LECTURE XXXVIl. 



Other with respect to the extent that they occupy, in ratios nearly analo- 

 ffous to those of the ascendino; scale of the minor mode in music. The 

 ohservations were, however, imperfect, and the analogy was wholly imagin- 

 ary. Dr. Wollaston has determined the division of the coloured image or 

 spectrum, in a much more accurate manner than had been done hefore: by 

 looking through a prism, at a narrow line of light, he produces a more effec- 

 tual separation of the colours, than can be obtained by the common method 

 of throwing the sun's image on a wall. The spectrum formed in this manner 

 tonsists of four colours only, red, green, blue, and violet, which occupy 

 spaces in the proportion of \6, 23, 36, and 25, respectively, making together 

 100 for the whole length; the red being nearly one sixth, the green and the 

 violet each about one fourth, and the blue more than one third of the length. 

 The colours differ scarcely at all in quality within their respective limits. 

 but they vary in brightness ; the greatest intensity of light being in that 

 part of the green, which is nearest to the red. A narrow line of yellow is 

 generally visible at the limit of the red and green, but its breadth scarcely 

 pxceeds that of the aperture by which the light is admitted, and Dr. Wollaston 

 attributes it to the mixture of the red with the green light. There are also 

 several dark lines crossing the spectrum within the blue portion and in its 

 neighbourhood, in which the continuity of the light seems to be ii\terrupted. 

 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 of the obliquity of the rays. The angu- 

 lar extent of the spectrum formed by a prism of crown glass is one 27th of 

 the deviation of the red rays; by a prism of flint glass, one 19th. (Plate 

 XXIX. Fig. 419.) 



In light produced by the combustion of terrestrial substances, the spectrum 

 is sometimes 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 

 composed 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 refraction. The electrical spai k furnishes also a light which is differently 

 divided in different circumstances. (Plate. XXIX. Fig. 420.) 



