Page:EB1922 - Volume 32.djvu/73

telescopic mast consists of 8 tubes. The lower one is attached to the carriage, and the upper one is pulled out as far as it will go and retained in position by catches before the mast is raised. The other six are connected to each other and to the lowest one by wire

cables and pulleys in such a way that when the cable which con- nects the two lowest tubes is wound in by means of a winch, each of the tubes except the fixed one will rise within the next one

FIG. 4

through the same distance. When erect, the mast is steadied by means of three guy ropes.

The details of the optical systems are as follows : The rays from a distant object after passing through a protecting window A (fig. 5)

are reflected by a mirror B down the centre of the conical casing which contains the upper optical system and is attached to the top of the mast. The two achromatic lenses, C and D, bring the rays to a focus on the plane surface of the large lens, E, forming an image there. Immediately above this plane surface and almost touching it is a system of wires which enables angular distances from the centre of the field to be read at the eyepiece below. The mirror can be elevated and depressed by means of a flexible shaft which passes up the centre of the mast and actuates gear attached to the mirror frame.

From the large lens, E, the rays pass through the open air for a considerable distance, depending upon how much the mast has been raised, to the lower optical system. Here they pass through the lenses and prism shown into one of the eyepieces, F. By moving the lens G up and down the image can be formed in the correct position for the eyepiece at all extensions of the mast.

There are three eyepieces which are mounted on a revolving sleeve in such a way that any_ one of them can be quickly brought into use, to give the magnification suitable to the height of the mast. (Low power from 3 to 8; medium from 5 to 14; high from 7 to 21.) Each eyepiece is provided with a dummy eyepiece which comes opposite to the eye which is not observing and permits of it being kept open. This lessens eyestrain. Coloured anti-glare glasses are provided.

(2) Submarine Periscopes. When a submarine is completely submerged the occupants are not able to see through the water except under very exceptional conditions. In the Mediterranean on a sunny day it is possible to see for several yards through the water at about 25 ft. below the surface. In the North Sea, and usually, it might be said that once the boat is submerged, direct observation through the water is impossible. In the very ear- liest submarines a cupola was built on the top of the hull, which was kept just above the surface when it was desired to take observations. To re- ducBj resistance, these cupolas were made tele- scopic in the French submarines "Gymnote"and the " Gustave Z6d6, " but the arrangement proved unsatisfactory. An optical tube replaced this cu- pola in the "Gustave Z6de," and comprised a short tube (on top of the submarine) with a lens to close the top end, which was kept just above the sur- face when running submerged. Horizontal rays of light entering at the top were reflected by a prism down the tube and focussed on to a sheet of paper in front of the helmsman inside the submarine. This gave him a limited view of what lay directly ahead. The word "periscope" was first applied to this instrument.

The modern submarine periscope consists essen- tially of a long tube, the top of which is just above the water when diving, while the lower end passes through a stuffing box on the shell of the boat into the control-room. The top is closed by a pressure-tight window, inside of which is a prism which reflects the light rays vertically down the tube to a prism at the bottom end, where they are reflected in a horizontal direction and focussed in an eyepiece attached to the bottom of the tube. Thus the commander can see what is happening on the surface when navigating the submarine some 2O ft. or mere below it.

The greater the depth of submergence the less the disturbance made by the submarine on the surface of the water, and the greater the immunity from gun-fire, ramming, etc.; also in a sea-way the deeper the submarine the more readily is it con- trolled. For these reasons the length of the peri- 5Jr7 FIG. 6 scope has steadily increased, and the dimensions **,*. of the upper end have as steadily decreased. In- creased length necessitated an increase in the diameter of the main tube to limit the amplitude of the vibrations caused by being pushed through the water. A typical instrument in the British navy was 30 ft. long, with a 5-9 in. diameter main tube, and the top 3 ft. of the upper tube 2 in. diameter. For the German " U " boats Messrs. Zeiss made a periscope 7 metres long, main tu^e 150 mm. (5-9 in.), and about 2 ft. 6 in. of the top tube 30 mm. (1-2 in.) diameter.

The main tube must be accurately machined as it has to be readily trained in its stuffing-box as well as be water-tight in all positions, through a considerable range of vertical travel. The modern practice is to take rapid observations rather than to keep the periscope above the water all the time. To facilitate this mechanical lifting, gear is provided which is readily controlled, and can raise or lower the periscope at a speed approaching 25 ft. per minute.

The field of view is usually about 40 at a magnification of 1-5. It is therefore necessary to train the periscope round when taking observations on different bearings. This can be done in two ways, either by rotating the optical train inside the main tube, or, as is more usually the case, rotating the whole periscope. With the increase in weight and size the effort required has increased, and power training has sometimes been necessary. Where possible,