By making a hole in the screen LM opposite any one of the colours of the spectrum, so as to allow that colour alone to pass--and by letting the colour thus separated fall upon a second prism--Newton found that the light of each of the colours was alike refrangible, because the second prism could not separate them into an oblong image, or into any other colour. Hence he called all the seven colours _simple_ or homogeneous, in opposition to _white_ light, which he called _compound_ or heterogeneous. With the prism which this philosopher used he found the lengths of the colours and spaces of the spectrum to be as follows: Red, 45; Orange, 27; Yellow, 40; Green, 60; Blue, 60; Indigo, 48; Violet, 80: or 360 in all. But these spaces vary a little with prisms formed of different substances, and as they are not separated by distinct limits, it is difficult to obtain any thing like an accurate measure of their relative extents. Newton examined the ratio between the sines of incidence and refraction of these decompounded rays (see p. 30,) and found that each of the seven primary colour-making rays, had certain limits within which they were confined. Thus let the sine of incidence in glass be divided into 50 equal parts, the sine of refraction into air of the _least_ refrangible, and the _most_ refrangible rays will contain respectively 77 and 78 such parts. The sines of refraction of all the degrees of _red_ will have the intermediate degrees of magnitude, from 77 to 77 one-eighth; _Orange_ from 77 one-eighth to 77 one-fifth; _Yellow_ from 77 one-fifth to 77 one-third; _Green_ from 77 one-third to 77 one-half; _Blue_ from 77 one-half to 77 two-thirds; _Indigo_ from 77 two-thirds to 77 seven-ninths; and _Violet_ from 77 seven-ninths to 78. From what has been now stated, it is evident that, in proportion as any part of an optic glass bears a resemblance to the form of a prism, the component rays that pass through it must be necessarily separated, and will consequently paint or tinge the object with colours. The edges of every convex lens approach to this form, and it is on this account that the extremities of objects when viewed through them are found to be tinged with the prismatic colours. In such a glass, therefore, those different coloured rays will have _different foci_, and will form their respective images at different distances from the lens. Thus, suppose LN (fig. 32.) to represent a double convex-lens, and OB an object at some distance from it. If the object OB was of a pure red colour, the rays proceeding from it would form a red image at RR; if the object was of a violet colour, an image of that colour would be formed at VV, _nearer_ the lens; and if the object was white or any other combination of the colour-making rays, those rays would have their respective foci at different distances from the lens, and form a succession of images, in the order of the prismatic colours, between the space RR and VV. [Illustration: _figure 32._]