HISTOBY OF PHYSICAL ASTEONOMY.

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unequal refrangibility of light, lie perceived that the existence of thatprinciple offered a serious obstacle to any further improvement of therefracting telescope. This great philosopher did not fail to make expe-riments on the passage of a ray of light through several contiguous media,in order to ascertain whether the different refractions might not correcteach other, so that the emergent ray, while deflected from its original-course,might still be free of colour. The results of his labours on this occasionserved, however, to convince him that no such compensation could beeffected. He found, in fact, that when the ray emerged in a directionparallel to its original course, it was invariably white; but when itemerged obliquely with respect to the direction of incidence, it becamegradually tinged with different colours in passing from the place of emer-gence *. Having satisfied himself in this manner that no combinationof substances could be devised which would be capable of refracting therays of light so as to produce a colourless image of an object, he seems tohave despaired of improving the refracting telescope by any other meansthan that of increasing the focal length of the object-glass.

It was reserved for Dollond j- to shew that the views which guidedNewton in arriving at this conclusion were erroneous, and to apply thetrue principle of physics to the construction of refracting telescopes freeof colour. This highly-gifted individual, although moving in a humblesphere of life, had devoted much attention to mathematical and physicalpursuits. He seems to have been originally led to consider the subject ofthe dispersion of light by the researches of Euler . In 1747 that distin-guished mathematician communicated a memoir to the Berlin Academy ofSciences , in which he attempted to shew that, by a combination of differentsubstances, it would be possible to construct object-glasses for telescopes,in which the effects arising both from spherical aberration and the unequalrefrangibility of light would be completely corrected. He remarked thatthe different humours of the human eye refracted the rays of light, so asto produce a porfectly-distinct image of an object, and he argued by ana-logy, that a similar effect would be produced by the object-glass of atelescope, provided a suitable combination of substances was employed inits construction. Proceeding upon a certain hypothetic law of the dis-persion of light, he accordingly devised a combination of hollow sphericallenses, enclosing water within their concave surfaces, which, he asserted,

* Optics, Book II. Part I. Exper. viii. Newton states that he made this experimentwith a prism glass and one of water, so adapting the two prisms that they refracted the lightequally in opposite directions. In such a case, although the rays of light emerged paral-lel to the direction of incidence, they ought also to have been coloured, as Dollond , uponrepeating the experiment in the following century, actually found them to be ; but New-ton asserts that he found the emergent rays to be white. Sir David Brewster states thatthe mistake of Newton arose from his having mixed a little sugar of lead with the water,in order to increase its refractive power, and that the high dispersive power of the ingre-dient caused the residuum of uncorrected colour to disappear (Life of Newton, p. 58).The same statement is made by Sir John Herschel , in his Treatise on Light, in theEncyclopaedia Metropolitana. Neither of the two distinguished philosophers just citedlias acquainted his readers with the source whence this explanation of Newton ’s mistakehas been derived. Newton himself makes no mention of his having added any ingre-dient to the water in the part of his work above referred to, where he alludes to theexperiment.

f John Dollond was born at Spitalfields, in London , in the year 1706. He was de-scended from French ancestors, who were compelled to quit Normandy upon the revo-cation of the edict of Nantes, by Louis XIV . In early life he worked at the loom, butin 1752 he joined his son as an optician. He died in 1761, having been struck withapoplexy, while engaged in an intense study of Clairaut ’s Theory of the Moon.

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