[Illustration: _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.