There are four types of refractive telescopes used for military purposes, viz:
- The astronomical.
- The terrestrial.
- The galilean.
- The prismatic.
[Figure 26] is a section of an astronomical telescope. The object glass (D) is a combination consisting of a double convex and a double concave lens cemented together with Canada balsam. The double concave lens is added to correct for chromatic aberration. The ocular (E) is a convex-concave lens.
Rays of light from some distant object are converged by the objective (D) and form an inverted image (ab) at the focal plane (F). The eye lens (E) receives the divergent pencils from a and b and bend them so that they enter the eye as if coming apparently from the direction of a′ b′ where the apparent image is seen. From the eyepiece (E) the rays emerge in a cone of pencils of light smaller than the pupil of the eye, which enables a telescope of this type to have a large field of view. The image, however, is inverted and the astronomical telescope in its original form is therefore not suitable for military purposes. In a modified form it is much used, as will be shown in a later paragraph.
Figure 26
[Figure 27] is a section of a terrestrial telescope much used for military purposes. Glasses of this type are quite generally known as "spyglasses."
As in the case of the astronomical telescope, the first inverted image ba is formed at the focal plane (F), and the first eyeglass converges these pencils to L. Instead of placing the eye at L, as in the astronomical telescope, the pencils are allowed to cross and fall on a second eyeglass, by which the rays of each pencil are converged to a point in the second erect image a′ b′, which image is viewed by means of the third and last eyeglass.