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.
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.
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.
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.
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.