ON PHYSICAL OPTICS. 343 



, It has generally been supposed, since the time of Newton, that when the 

 rays of light are separated as completely as possible by means of refrac- 

 tion, they exhibit seven varieties of colour, related to each other with 

 respect to the extent that they occupy, in ratios nearly analogous to those 

 of the ascending scale of the minor mode in music. The observations 

 were, however, imperfect, and the analogy was wholly imaginary. Dr. 

 Wollaston * has determined the division of the coloured image or spectrum, 

 in a much more accurate manner than had been done before : by looking 

 through a prism, at a narrow line of light, he produces a more effectual 

 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 

 consists of fo.ur colours only, red, green, blue, and violet, which occupy 

 spaces in the proportion of 16, 23, 36, and 25, respectively, making toge- 

 ther 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 

 I breadth scarcely exceeds that of the aperture by which the light is ad- 

 , mitted, and Dr. Wollaston attributes it to the mixture of the red with the 

 green light. There are also several dark lines t crossing the spectrum 

 within the blue portion and 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 varia- 

 tions of the obliquity of the rays. The angular 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 spec- 

 trum is sometimes still more interrupted ; thus, the bluish light of the 



* Ph. Tr. 1802, p. 365. 



f This fact did not excite the attention which it merited at the time of its dis- 

 covery. Several years afterwards, M. Fraunhofer, of Munich, by viewing the 

 spectrum formed from a narrow line of solar light, when in its purest state, at 

 the angle of minimum deviation, discovered that it was crossed by a very great 

 number of dark lines, not separating different colours, but mixed up with them, 

 without any order. In solar light they are nearly 600 in number, and with 

 the same kind of light always retain the same places, but are very different for dif- 

 ferent kinds of light ; and even that of the sun, after it has been transmitted through 

 nitrous acid gas, exhibits very different lines from what it did previously. By far the 

 readiest mode of viewing such lines, is to cause sun-light to pass through a bottle of 

 this gas before it falls on the prism. Since these lines always retain their places in 

 the spectrum, they afford the most accurate method of determining the refractive 

 and dispersive powers of bodies, to which purpose Fraunhofer himself applied them. 

 See Fraunhofer, Bestimmung des Brechungs und F-arbenszerstreuungs-vermogens 

 verschiedener Glasarten. Miincher Akad. Abhand. 1821,' xxii. Brewster's Obser- 

 yations on the Lines produced by the Earth's Atmosphere and by the Action of 

 Nitrous Acid Gas, Tr. Roy. Soc. Ed. xii. 519. Ed. Jour, of Sci. No. XV. 7. Mil- 

 ler, ibid. ii. 381. Rudberg, Pogg. Ann. xxxv. 523. Wheatstone (Electrical Light), 

 Ph. Mag. vii. 299. 



