When the axis of the eye-lens coincides with that of the object-glass, and the focal point of the eye-lens is coincident with the principal focus of the object-lens, parallel rays.
The magnifying power obviously depends on the proportion of the focal length of the object-lens to that of the eye-lens, that is, magnifying power where F is the focal length of the object-lens and e that of the eye-lens.
Also the diameter of the pencil or parallel rays emerging from the eye-lens to the diameter of the object-lens inversely as, the magnifying power of the telescope.
The substitution of a positive or negative eye-piece for the simple convex or concave eye-lens, and of an achromatic object-glass for the simple object-lens, transforms these, early forms into the modern achromatic telescope.
The materials for it were rapidly accumulated by the use of an objective prism, that is, of a prism placed in front of, instead of behind the object-lens, by which means the spectra of all the stars in the field, to the number often of many score, imprinted themselves simultaneously on the sensitive plate.
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