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. [Illustration: _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. [Illustration: _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. [Illustration: _Figure 28_] PORRO PRISM FIELD GLASSES AND TELESCOPES.