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.
figure 32.
figure 33.
This may be illustrated by experiment in the following manner. Take a card or slip of white pasteboard, as ABEF, (fig. 33.) and paint one half ABCD red, the other half CF, violet or indigo; and tying black threads across it, set it near the flame of a candle G, then take a lens HI, and holding a sheet of white paper behind it, move it backwards and forwards upon the edge of a graduated ruler, till you see the black threads most distinctly in the image, and you will find the focus of the violet FE, much nearer than that of the red AC, which plainly shows that bodies of different colours can never be depicted by convex-lenses, without some degree of confusion.
The quantity of dispersion of the coloured rays in convex lenses depends upon the focal length of the glass; the space which the coloured images occupy being about the twenty-eighth part. Thus if the lens be twenty-eight inches focal distance, the space between Rr and Vv (fig 32) will be about one inch; if it be twenty-eight feet focus, the same space will be about one foot, and so on in proportion. Now, when such a succession of images formed by the different coloured rays, is viewed through an eye-glass, it will seem to form but one image, and consequently very indistinct, and tinged with various colours, and as the red figure Rr is largest, or seen under the greatest angle—the extreme parts of this confused image will be red, and a succession of the prismatic colours will be formed within this red fringe, as is generally found in common refracting-telescopes, constructed with a single object-glass. It is owing to this circumstance that the common refracting telescope cannot be much improved without having recourse to lenses of a very long focal distance; and hence, about 150 years ago, such telescopes were constructed of 80, and 100, and 120 feet in length. But still the image was not formed so distinctly as was desired, and the aperture of the object-glass was obliged to be limited. This is a defect which was long regarded as without a remedy; and even Newton himself despaired of discovering any means by which the defects of refracting telescopes might be removed and their improvement effected. This, however, was accomplished by Dollond to an extent far surpassing what could have been expected, of which a particular account will be given in the sequel.
It was originally remarked by Newton, and the fact has since been confirmed by the experiments of Sir W. Herschel, that the different-coloured rays have not the same illuminating power. The violet rays appear to have the least illuminating effect; the indigo more, and the effect increases in the order of the colours,—the green being very great; between the green and the yellow the greatest of all; the yellow the same as the green; but the red less than the yellow. Herschel also endeavoured to determine whether the power of the differently-coloured rays to heat bodies, varied with their power to illuminate them. He introduced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in succession, and observed the heights to which it rose in a given time. He found that their heating power increased from the violet to the red. The mercury in the thermometer rose higher when its bulb was placed in the Indigo than when it was placed in the violet, still higher in blue, and highest of all at red. Upon placing the bulb of the thermometer below the red, quite out of the spectrum, he was surprised to find that the mercury rose highest of all; and concluded that rays proceed from the sun, which have the power of HEATING, but not of illuminating bodies. These rays have been called invisible solar rays. They were about half an inch from the commencement of the red rays; at a greater distance from this point the heat began to diminish, but was very perceptible even at the distance of 1½ inch. He determined that the heating power of the red to that of the green rays, was 2¾ to 1, and 3½ to 1, in red to violet. He afterwards made experiments to collect those invisible calorific rays, and caused them to act independently of the light, from which he concluded that they are sufficient to account for all the effects produced by the solar rays in exciting heat; that they are capable of passing through glass, and of being refracted and reflected, after they have been finally detached from the solar beam.