Since a diminished angular magnification is to be obtained, the negative lens c of the Galilean telescope precedes the objective d. When the high power is in use the lenses c and d are swung to one side, as at and against the side of the tube. Fig. 2 shows the paths of two beams through the low power system, that along the axis having the full cross section admitted by the lenses g and h and the other from a point at the edge of the field showing by its smaller cross section the diminution in illumination which takes place.
Fig. 3 shows the upper end of a Barr and Stroud periscope of the sky-searching type. The lower end is illustrated in fig. 4.
To enable the observer's line of sight to be directed to any part of the horizon, the instrument is rotated about its axis by the handles shown in fig. 4. When not in use the handles can be folded out of the way. The eyepiece (between the handles in fig. 4) has a rubber cushion which serves as a protective guard. In focus with the image of external objects are seen the graduations of vertical and horizontal scales. These scales enable the observer to make a fair estimate of the apparent sizes of objects and approximate distances. The bearing scale is projected into the field of view by means of a small prismatic telescope. As a result of this projec tion the bearing scale appears in the top of the field of view so as to be readily seen by the observer without taking the eye from the eyepiece.
In addition to reading the bearing it is important that the observer should be able to bring any object into the centre of the field of view without removing his eye from the eyepiece.
This involves control over the angle of elevation of the line of sight and ability to adjust the focus. By rotating one of the han dles about its axis the top prism of this periscope may be adjusted for elevation. The two milled heads, which are shown projecting from the bottom of the periscope in fig. 4, control not only the entire focusing adjustment but also regulate the light filters. The change of magnification or power is brought about by rotating the second handle.
In ordinary use the low power telescope is employed, and a change is only made to the high power when it is desired to ob serve an object more minutely than is possible with the low power. As the low power embraces an angular area sixteen times as great as the high power, large regions can be observed much more rapidly than with the high power.
Special provision has to be made for desiccating the air in the interior of the periscope from time to time. The material of the tubes must be non-magnetic and not liable to be corroded by sea water. Very slight action of this kind is serious, since it causes the tube to work stiffly in its watertight gland.
The principles of construction of periscopes for use on land are the same as those described for naval instruments. The con ditions to be satisfied are in general less stringent than in sub marine periscopes, and in the absence of important special fea tures it is unnecessary to give illustrations of these instruments. See the article "Periscope" in Dictionary of Applied Physics, vol. 4. (T. Sm.)