At the time of Sir William Herschel the practical difficulties in constructing large achromatic lenses led to the adoption by him of reflectors beginning with small apertures of six inches to a foot, and increasing till he obtained one of four feet in diameter and forty-six feet focal length. This has been surpassed by Lord Rosse, whose well-known telescope is six feet diameter, and fifty-three feet focal length. Mr. Lassell, Mr. De La Rue, M. Foucault and Mr. Grubb, have also more recently succeeded in bringing reflectors to great perfection. How the work has been done will be fully stated in the sequel. CHAPTER IX. EYEPIECES. We have considered the telescope as a combination of an object-glass and eyepiece in the one case, and of a speculum and eyepiece in the other; that is to say, we have discussed the optical principles which are applied in the construction of refracting and reflecting telescopes, the telescope being taken as consisting of an object-glass or speculum and an eyepiece of the most simple form, viz., a simple double convex lens. We must now go into detail somewhat on the subject of eyepieces, and explain the different kinds. It will be recollected that when we spoke of the object-glass, its aberration, both chromatic and spherical, was mentioned. Now every ordinary lens has these errors, and eyepieces must be corrected for them, but this is not done in exactly the same way as with object-glasses. In the case of eyepieces the error is corrected by using two lenses of such focal lengths or at such a distance apart that each counteracts the defects of the other; not by using two kinds of glass as in the case of the object-glass, but by so arranging the lenses that the coloured rays produced by the first lens shall fall at different angles of incidence on the second and become recombined. [Illustration: FIG. 60.—Huyghens’ Eyepiece. ]