We now know how these difficulties can be, to a great extent, overcome, by employing for the objective a composite lens made of two pieces of glass possessing different qualities.  To these achromatic object glasses, as they are called, the great development of astronomical knowledge, since Newton’s time, is due.  But it must be remarked that, although the theoretical possibility of constructing an achromatic lens was investigated by Newton, he certainly came to the conclusion that the difficulty could not be removed by employing a composite objective, with two different kinds of glass.  In this his marvellous sagacity in the interpretation of nature seems for once to have deserted him.  We can, however, hardly regret that Newton failed to discover the achromatic objective, when we observe that it was in consequence of his deeming an achromatic objective to be impossible that he was led to the invention of the reflecting telescope.  Finding, as he believed, that the defects of the telescope could not be remedied by any application of the principle of refraction he was led to look in quite a different direction for the improvement of the tool on which the advancement of astronomy depended.  The refraction of light depended as he had found, upon the colour of the light.  The laws of reflection were, however, quite independent of the colour.  Whether rays be red or green, blue or yellow, they are all reflected in precisely the same manner from a mirror.  Accordingly, Newton perceived that if he could construct a telescope the action of which depended upon reflection, instead of upon refraction, the difficulty which had hitherto proved an insuperable obstacle to the improvement of the instrument would be evaded.

[Plate:  Sir Isaac Newton’s little reflector.]

For this purpose Newton fashioned a concave mirror from a mixture of copper and tin, a combination which gives a surface with almost the lustre of silver.  When the light of a star fell upon the surface, an image of the star was produced in the focus of this mirror, and then this image was examined by a magnifying eye-piece.  Such is the principle of the famous reflecting telescope which bears the name of Newton.  The little reflector which he constructed, represented in the adjoining figure, is still preserved as one of the treasures of the Royal Society.  The telescope tube had the very modest dimension of one inch in diameter.  It was, however, the precursor of a whole series of magnificent instruments, each outstripping the other in magnitude, until at last the culminating point was attained in 1845, by the construction of Lord Rosse’s mammoth reflector of six feet in aperture.