HISTORY OF PHYSICAL ASTRONOMY.

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be received by a concave parabolic speculum, and, after forming by re-flexion an image of the object at the focus, should hence diverge and fallupon a smaller concave speculum of an elliptic figure having the samefocus as the larger one. The rays would now be reflected by the ellipticspeculum, so as to form a second image of the object near the anteriorsurface of the larger, or parabolic, speculum ; and this image might finallybe viewed by the observer through an eye-lens placed behind the latterspeculum, which was to be perforated for that purpose. Gregory expectedthat, by substituting the principle of the reflexion of light for that of re-fraction, so as to get rid of the effect of spherical aberration, the imageof the object would be rendered much more distinct. When he was inLondon , about the year 1664, he employed an optician to execute a tele-scope for him according to the above design, but the attempt to givea true parabolic figure to the larger speculum proved a failure; and,although he subsequently tried to effect the same object by means of aspherical speculum, yet, in this instance also, the result was so unsatis-factory that he seems to have been discouraged from prosecuting the matterfurther.

It appears that, before the publication of the Optica Promota, Mer-senne had already suggested the idea of a reflecting telescope. Thishe did originally in a letter to Descartes written about the year 1639 *,and, afterwards, in his Catoptrics, which was published in 1651. Descartes offered several objections to Mersenne ’s proposal, and no attempt wasmade to carry it into effect. It has been said, by Fontenelle, that FatherZucci was acquainted with the reflecting telescope as early as 1618.Indeed, Montucla has justly remarked, that as soon as it was known thatthe image of an object might be formed in the focus of a concave mirror,the idea of applying the principle of reflexion to the construction of tele-scopes (after the invention of refracting telescopes) was a very obvioussuggestion. It is impossible to ascertain whether Gregory was indebtedto any previous writer for the hint of the reflecting telescope, or not; but,at any rate, it must be allowed that he was the first person who gave acomplete explanation of the construction of such a telescope. Moreover,he has the merit of having suggested a form of the instrument which hasbeen in use ever since, and which, as far as ordinary purposes are con-cerned, has been found more convenient in practice than any other thathas subsequently been devised.

The first person who actually executed a reflecting telescope was New-ton. As soon as that philosopher discovered the unequal refrangibility oflight, about the year 1666, it occurred to him that any further improve-ment of the refracting telescope was impossible, and he was led, in con-sequence, to consider the application of the principle of reflexion to thesame object. Finding that for the different rays of the prism, the angleof reflexion was in each case equal to the angle of incidence, he per-ceived that, by means of this property of light, optical instruments mightbe brought to any degree of perfection, provided a substance could befound which reflected a sufficient quantity of light, while at the same timeit was capable of being highly polished, and of receiving a true parabolicfigure. But the difficulties which stood in the way of accomplishing thefulfilment of these conditions, were very great; nay, they almost seemedto him to be insuperable, when he recollected, that every irregularity in areflecting surface would make the rays deviate five or six times more out

* “ Lettres de Descartes,” tom. ii., Nos. 29, 32. 8vo, Paris, 1659.