Method of measuring small angular distances between heavenly bodies by the size of the aperture of the telescope.method of measuring distances remains for inquiry, and this we shall accomplish by the following contrivance:—
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For the sake of being more easily understood, I will suppose a tube a b c d.[1] Let e be the eye of the observer; then, when there are no lenses in the tube rays from the eye to the object f g would be drawn in the straight lines e c f, e d g, but when the lenses have been inserted, let the rays go in the bent lines e c h, e d i,—for they are contracted, and those which originally, when unaffected by the lenses, were directed to the object f g, will
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The line c h in Galileo's figure represents the small pencil of rays from n which, after refraction through the telescope, reach the eye e. The enlarged figure shows that if o p be the radius of the aperture employed, the point h of the object would be just outside the field of view. The method, however, is at best only a very rough one, as the boundary of the field of view in this telescope is unavoidably indistinct.
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