Figure 27
Terrestrial telescopes have a comparatively small field of view. The barrels of this telescope are necessarily long on account of the additional lenses.
GALILEAN FIELD GLASSES AND TELESCOPES.
[Figure 28] is a section of a Galilean telescope which differs from the astronomical telescope in having a double concave instead of a double convex, eyepiece or ocular.
In this telescope the rays from an object are converged by the object glass (O) and would normally focus at the focal plane (C) and there form the inverted image ba were it not that the double concave eyeglass or ocular (D) is so located in the barrel of the telescope as to intercept the pencils before they are focused. This double concave eyeglass diverges these pencils and forms a magnified erect image a′ b′ apparently at E. Due to the diverging action of this concave eye lens, the cone of pencils entering the eye is larger than the pupil of the eye, and therefore but a small part of the field gathered by the object glass is utilized by the eye, which causes telescopes of this type to have a comparatively small field of view.
Figure 28