But the fame of Dollond principally rests upon his invention of _achromatic_, or colourless telescopes, in which the surrounding fringe of colours was destroyed, which had rendered indistinct the images formed in all refracting telescopes previously constructed. He was led to this practical result by the discovery of a principle in optics, that the _dispersion_ of light in passing through a refracting medium, that is, the greater or less length through which the coloured _spectrum_ is scattered, is not in proportion to the _refraction_, or angle through which the rays are bent out of their course. Newton asserted that he had found by experiments, made with water and glass, that if a ray of light be subjected to several refractions, some of which correct the rest, so that it emerges parallel to its first direction, the dispersion into colours will also be corrected, so that the light will be restored to whiteness. This is not generally true: it is true if one substance only be employed, or several which have the same, or nearly the same, _dispersive power_[2]. Mr. Peter Dollond afterwards satisfactorily explained the reason of Newton’s mistake, by performing the same experiment with Venetian glass, which, in the time of the latter, was commonly used in England; from which he found that the fact stated by Newton was true, as far as regarded that sort of glass. Had Newton used flint glass, he would have discovered that dispersion and refraction are not necessarily corrected together: he would then have been led to the difference between refractive and dispersive power, and would have concluded from his first experiment that Venetian glass and water have their dispersive powers very nearly equal. As it was, he inferred that the refracting telescope could never be entirely divested of colour, without entirely destroying the refraction, that is, rendering the instrument no telescope at all; and, the experiment being granted, the conclusion was inevitable. It is well known that he accordingly turned his attention entirely to the reflecting telescope.
See Penny Cyclopædia, article Achromatic, for this and other terms employed in this life.
In 1747, Euler, struck by the fact that the human eye is an achromatic combination of lenses, or nearly so, imagined that it might be possible to destroy colour by employing compound object-glasses, such as two lenses with an intermediate space filled with water. In a memoir addressed to the Academy of Berlin, he explained his method of constructing such achromatic glasses, and proposed a new law of refrangibility, different from that of Newton. He could not, however, succeed in procuring a successful result in practice. Dollond, impressed with the idea that Newton’s experiment was conclusive, objected to Euler’s process in a letter to Mr. Short; which the latter persuaded the author to communicate, first to Euler, and then, with his answer, to the Royal Society. Assuming Newton’s law, Dollond shows that Euler’s method would destroy all refraction as well as dispersion. The latter replies, that it is sufficient for his purpose that Newton’s law should be _nearly_ true; that the theory propounded by himself does not differ much from it; and that the structure of the eye convinces him of the possibility of an achromatic combination. Neither party contested the general truth of Newton’s conclusion.