344 LECTURE XXXVII. 



lower part of a flame of a candle is separated by refraction into five parcels 

 cf 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 spark furnishes also 

 a light which is differently divided in different circumstances. (Plate 

 XXIX. Fig. 420.) 



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 

 augmented in the same proportion, and each portion encroaches on the 

 neighbouring colours, and is mixed with them : so that the red is suc- 

 ceeded 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 prismatic spectrum is commonly exhibited. (Plate 

 XXIX. Fig. 421.) 



When the beam of light is so much enlarged as to exceed the angular 

 magnitude of the spectrum, it retains its whiteness in the centre, and is ter- 

 minated by two different series of colours at the different ends. These series 

 are still divided by well marked lines : on the one hand the red remains 

 unmixed ; the space belonging to the green and blue becomes a greenish 

 yellow, nearly uniform throughout, and here the appearance of colour ends, 

 the place of the violet being scarcely distinguishable from the neighbouring 

 white light : on the other hand, the space belonging to the red, green, and 

 blue of the simple spectrum, appears of a bluish green, becoming more and 

 more blue till it meets the violet, which retains its place without alteration. 

 This second series is also the same that accompanies the limit of total 

 reflection at the posterior surface of a prism. (Plate XXIX. Fig. 422.) 



Sir Isaac Newton observed that the effect of white light on the sense of 

 sight might be imitated by a mixture of colours taken from different parts 

 of the spectrum, notwithstanding the omission of some of the rays naturally 

 belonging to white light. Thus, if we intercept one half of each of the four 

 principal portions into which the spectrum is divided, the remaining halves 

 will still preserve, when mixed together, the appearance of whiteness ; so 

 that it is probable, that the different parts of those portions of the spectrum, 

 which appear of one colour, have precisely the same effect on the eye. It 

 is certain that the perfect sensations of yellow and of blue are produced 

 respectively, by mixtures of red and green and of green and violet light, and 

 there is reason to suspect that those sensations are always compounded of 

 ( the separate sensations combined ; at least, this supposition simplifies the 

 theory of colours : it may, therefore, be adopted with advantage, until it be 

 found inconsistent with any of the phenomena ; and we may consider white 

 light as composed of a mixture of red, green, and violet only, in the pro- 

 portion of about two parts red, four green, and one violet, with respect to 

 the quantity or intensity of the sensations produced.* 



* So WUnsch, Versuche iiber die Farben, Leipz. 1792. Mayer, in an essay De 

 Affinitate Colorum, pub. 1722, refers all colours to red, yellow, and blue : and thk 

 is the more common hypothesis. See Guyot, Recreations, Par. 1769. Goethe, Far- 

 benlehre, 1810. Brewster, Tr. Roy. Soc. Ed. xii. 123. Nollet, Lesons de Phy- 

 sique, v. 388, considers the three colours to be orange, green, and indigo. 



