Of the magnifying power of telescopes, for the rays emerging from the eye-piece when adjusted for distinct vision to be parallel.
But although the argument from gratings is instructive and convenient in some respects, its use has tended to obscure the essential unity of the principle of the limit of resolution whether applied to telescopes or microscopes.
Until Newton's discovery of the different refrangibility of light of different colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature.
(2) Telescopes for General P urposes.
When equatorial mountings for telescopes became more general, no filar micrometer was considered complete which was not fitted with a position circle.'