352

PERISCOPES

roof combination laterally, so that the light always meets the surfaces in the same order. Another case in which a roof combination

FIG. 4.

may be used is when an inverting telescope is employed with the observer facing the view. If the roof prism replace the lower plane prism of Fig. 1, the lateral inversion it causes compensates the inversion of the telescope in the horizontal plane, but the vertical inversion is uncorrected. The substitution at the upper end of the periscope of two mirrors inclined to one another in the vertical plane at an angle of 45° for the single mirror is a possible method of completing the rectification of the image (Fig. 5). An alternative of much ]?IQ. 5. importance consists in

the introduction of an

inverting isosceles prism. With this combination an erect image may be secured conveniently whatever the angle between the line of sight and the direction of observation

important that the directions of sight and of observation should be identical, and as the use of roof reflectors is not free from difficulties erecting telescopes become necessary.

§ (3) FIELD OF VIEW AND ILLUMINATION. — From the point of view of light deflection then the periscope offers 'no new problem at all for consideration. Optically its special characteristic is that, when the extent of the field of view required and the amount of light necessary from each part of the field are considered, the instrument is of such unusual length that special forms of construction are essential to enable these particular features to be realised in an acceptable manner.

(i.) Simple Periscopes. — Suppose that a distant view is seen through a long tube of length I and diameter d. The visible field has an angular diameter dfl if the observer's eye, situated in the position marked E in the figures, is stationary and the diameter of the pupil is small in comparison with d (Fig. 7). If a lens of dia-

FIG. 6.

f~ length 1/6 is placed in the tube at one-third its length from

the observer's end, an inverted image of the distant view of angular diameter 3d/l will be seen at the principal focus of the lens which is at a distance Z/6 from the observer's eye (Fig. 8). As

PIG. 7.

may be. The best known instrument of this kind has an upper isosceles right-angled prism, a lower roof prism, and an isosceles inverting prism placed along the vertical axis of the

far as the field of view is concerned the eye is virtually placed at e, the image of E formed by the lens. As exception is taken to a system such as the above, which necessitates focussing

telescope, so mounted that it rotates through half the relative displacement of the upper and lower prisms. This arrangement is illustrated in Fig. 6. In submarine periscopes it is considered

the eye on a fixed near plane, the single lens must be replaced by a telescopic system. The simplest consists of two single lenses of equal focal length with their separation double the focal length of either. If these are placed