274 FERGUSON'S LECTURES. 



LECT. into an orange colour, as they are nearer the line m: 

 ,x"v-^ the next will be of a fine orange colour at m, and from 

 thence degenerate into a yellow colour towards /: at / 

 they will be of a fine yellow, which will incline towards 

 a green, more and more, as they are nearer and 

 nearer k: at A; they will be a pure green, but from 

 thence towards i they will incline gradually to a blue : 

 at i they will be a perfect blue, inclining to an indigo 

 colour from thence towards h : at h they will be quite 

 the colour of indigo, which will gradually change 

 towards a violet, the nearer they are to g : and at g they 

 will be of a fine violet colour, which will incline gra- 

 dually to a red as they come nearer to /, where the 

 coloured image ends. 



There is not an equal quantity of rays in each of 

 these colours ; for, if the oblong image p q be divided 

 into 360 equal parts, the red space R will take up 45 

 of these parts ; the orange O, 27 ; the yellow Y, 48 ; 

 the green G, 60 ; the blue B, 60 ; the indigo 7, 40 ; and 

 the violet V, 80 : all which spaces are as nearly pro- 

 portioned in the figure as the small space p q would 

 admit of. 



If all these colours be blended together again, they 

 will make a pure white ; as is proved thus. Take away 

 the paper on which the colours p q fell, and place a 

 large convex glass D in the rays /, g, h, &c which 

 will refract them so, as to make them unite and cross 

 each other at W: and if a white paper be placed to 

 receive them, they will excite the idea of a strong lively 

 white. But if the paper be placed farther from the 

 glass, as at r s, the different colours will appear again 

 upon it, in an inverted order, occasioned by the rays 

 crossing at W. 



As white is a composition of all colours, so black is 

 a privation of them all, and therefore, properly no 

 colour. 



Let two concentric circles be drawn on a smooth 



