ON PHYSICAL OPTICS. 439 



If the breadth of the aperture viewed through a prism is somewhat in- 

 creased, the space occupied by each variety of hght in tiie spectrum is aug- 

 mented in the same proportion, and each portion encroaches on the neigh-, 

 bouring 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 prismatic spectrum is commonly exhibited. (Plate XXIX. Fig. 

 421.) 



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

 nitude of the spectrum, it retains its whiteness in the centre, and is term i-" 

 nated 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 pf 

 the violet being scarcely distinguishable from the neighbouring white light:- 

 on the other hand, the space belonging to the red, green, and blue, of thft 

 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 pos- 

 terior surface of a prism. (Plate XXIX. Fig. 422.) 



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

 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 naturallyl 

 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 respec- 

 tively, 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 

 aensations 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 w^ may consider. white light gis composed of 



