Sighting device

“ Such apparatus is essentially appurtenant to large instruments only, say of not less than 12′′ aperture and preferably much more. The eye is enormously more sensitive as a detector of radiant energy than any device of human contrivance, and thus small telescopes can be well used for visual photometry, the bigger instruments having then merely the advantage of reaching fainter stars. ”

“ The eyepiece of a telescope is merely an instrument for magnifying the image produced by the objective or mirror. If one looks through a telescope without its eyepiece, drawing the eye back from the focus to its ordinary distance of distinct vision, the image is clearly seen as if suspended in air, or it can be received on a bit of ground glass. ”

“ The power of a telescope or a field glass can more accurately be measured by means of a dynameter, which is a microscope which can be fitted over the eyepiece end of the instrument, and which magnifies the image. ”

“ I followed Jim in; he closed the door and started the air conditioner.
"Here, First Mortgage," he said as he turned from the control board and faced me, "here are the fluoroscopic screens. They are arranged in a bank, so that you can keep an eye on all of them readily. Beneath each telescope is an automatic one-pounder gun with its mount geared to the telescope and the light, so that the gun bears continually on the point in space represented by the center of the fluoroscopic screen which belongs to that light. ”

“ To find a signalman near any known station, note with the unaided eye some prominent landmark near which the looked-for person or object is supposed to be, and direct the telescope upon the place, as sight is taken over a gun barrel, covering the object; if the eye is now placed at the eyeglass of the telescope, the prominent or directing landmark will be found in the field of view. ”

“ Always pull out or push in the draws of your telescope with a clock-wise circular motion, and keep them slightly lubricated. 4. Always carry your telescope slung on your body. If you take it off and let it travel in a lorry or car the jolting will almost certainly ruin it. 5. Always use a rest when observing. 6. When looking into the sun, make a sun-shade nine inches or a foot long, to fit on the short sun-shade of the telescope. This will give you great assistance when the sun is over the German lines. ”

“ One instrument being explained to us is what we might call an all-seeing eye which is part of the radar equipment as we know it. This 'eye' can see for five thousand miles ahead and can detect any kind of storm, any object, or any elemental condition that might be approaching in space, including heavy fog which it can also penetrate. ”

“ Field glasses with low magnifying power, which are usually preferred by ordinary observers, have their chief value in the comparatively extensive field of view; they should be used to observe extensive movements, where large tracts of country must be taken in one field of view or in sweeping the landscape to find the tents of the enemy, their wagons, etc., or other objects, to be afterwards more closely examined with the telescope. ”

“ We require to have in the telescope something, say two wires crossed, placed in the field of view—in the round disc of light we see in a telescope owing to the construction of the diaphragm—so as to be seen together with any object. In the chapter on eyepieces it was shown that we get at the focus the image of the object; and as that is also the focus of the eyepiece, it is obvious that not only the image in the air, as it were, but anything material we like to put in that focus, is equally visible. ”

“ Over 50 per cent. of the telescopes in use, in or about the front line, have been scratched more or less badly, owing to the neglect of this simple precaution.
Special attention should be paid to the cleaning of the objective lens, which is liable to become covered with dust owing to its position in the telescope and the opening and closing of the draws. ”

“ The pointing of airplane cameras is therefore akin to the sighting of great guns. While the observer may perform the actual operation of taking the picture, the responsibility for covering the objective rests with the pilot. Teamwork counts equally with tools. Airplane camera sights may accordingly be divided into two classes: sights attached to the camera, for use principally with hand-held apparatus, and sights attached to the plane, for the use of pilot, of observer, or of both. ”

“ But on land such big range-finders cannot conveniently173 be used; they are too bulky. When it is necessary to get the range of a very distant object, two observers are used who are stationed several hundred yards apart. These observers have telescopes which they bear upon the object, and the angle through which they have to turn the telescope is reported by telephone to the battery, where, by a rapid calculation, it is possible to estimate the exact position of the target. Then the gun is moved up or down, to the right or to the left, according to the calculation. ”

“ When the power of a telescope is required, the person must measure the clear aperture of the object-glass, then holding the pearl dynameter next the eye-glass, let him observe how many divisions the small circle of light occupies, when the instrument is directed to a bright object. ”

“ Fixed parallel to the axis of the larger telescope is the finder, a small telescope of low power and large field, used for finding celestial objects not easily visible to the naked eye. It is so arranged, that when the object is found and carried to its centre, it is also in the centre of the field of the larger instrument. The handle and the two toothed wheels serve to raise or lower the telescope, which is movable on the horizontal axis, which supports it in front, so that it may be directed to any part of the heavens the observer may desire. ”

“ Without a good eyepiece the best telescope will not perform well. The simplest of all eyepieces is a single double-convex lens. With such a lens the magnifying power of the telescope is measured by the ratio of the focal length of the objective to that of the eye lens. ”

“ Any telescope can be made to magnify a thousand times, say, but it does not follow that you will be pleased with the result. The magnifying depends partly on the big lens or mirror about which we have been talking and partly on the "eyepiece," the little lens you put your eye to. If the object is not magnified enough you can take out this little eyepiece and put in another which magnifies more. Perhaps you have had your photograph taken and "enlarged": something of the same kind happens with a telescope, the big lens or mirror makes a little image and the eyepiece enlarges it. ”

“ Point the telescope toward the clear sky, focusing for a distant object so that the emergent pencil is sharply defined at the ocular, and then measure its diameter by the help of a fine scale and a pocket lens, taking care that scale and emergent pencil are simultaneously in sharp focus and show no parallax as the eye is shifted a bit. This bright circle of the emerging beam is actually the projection by the eye lens of the focal image of the objective aperture. ”

“ In the use of the telescope for photography, as we shall see, the system of bringing the mass of the instrument to complete rest before exposing the sensitive plate, and only driving that plate itself by a clock, is always adopted. ”

“ As regards the rest of the eyepiece equipment the observer may well suit his own convenience and resources. Usually one ocular of about half the maximum power provided will be found extremely convenient and perhaps oftener used than either the high or low power. Oculars of intermediate power and adapted for various purposes will generally find their way into any telescopic equipment. And as a last word do not expect to improve bad conditions by magnifying. If the seeing is bad with a low power, cap the telescope and await a better opportunity. ”

“ The Telescope, as its name implies, is an instrument employed for viewing distant objects. [2] It aids the eye in two ways; first, by enlarging the visual angle under which objects are seen, and, secondly, by collecting and conveying to the eye a much larger amount of the light that emanates from the object, than would enter the naked pupil. A complete knowledge of the telescope cannot be acquired, without an acquaintance with the science of optics ”

“ Focus the telescope for distant vision, align the telescope with the collimator, and move the slit till it (the slit) is seen distinctly in the telescope; then put the prism in place approximately, and move it round its axis until a position is found where (supposing the light of the sun or diffused daylight to be under examination) the lines in the green part of the spectrum are seen at their best advantage. It must be remembered that a slight change of focus is necessary for every color. ”

“ When the task was assumed of producing a new and improved telescope, it was observed that an ordinary telescope, built up of lenses, is inconveniently long and heavy in comparison with its magnifying power. ”

“ The _refractor_ is the ordinary, familiar type of telescope. It consists, essentially, of a large lens at one end of a tube, and a small lens, called the eye-piece, at the other. The function of the large lens is to act as a sort of gigantic eye. It collects a large amount of light, an amount proportional to its size, and brings this light to a focus within the tube of the telescope. ”

“ Fig. 59.—How to see through a Brick. horizontal ray, meeting the mirror at A, is consequently bent downwards to C, then travels horizontally to D, when it is reflected upwards to B, in which it is seen by the eye. Of course a simple tube without any lenses at all would serve the same purpose, but the fact of its being a telescope serves to distract the attention of the too curious observer.
Another illusion of the same kind is often practised at fancy fairs and bazaars, when a spectator looking into what he supposes to be an ordinary looking-glass, sees his companions instead of himself. ”

“ The ordinary Telescope consists of an object-glass and an eye-lens, with two intermediates to bring the object into an erect position. A lens brings it near to us, and a magnifier enlarges it for inspection. ”

“ The telescope is an artificial organ of sight, but it sees not. The eye is a natural organ of sight, by which we see; but the natural organ sees as little as the artificial.
The eye is a machine, most admirably contrived for refracting the rays of light, and forming a distinct picture of objects upon the retina; but it sees neither the object nor the picture. It can form the picture after it is taken out of the head, but no vision ensues. ”

“ The telescope is an optical instrument based on an object glass or reflector to form a real image of a real and distant object, and of an ocular to magnify and view the image. Telescopes are classified as refracting or reflecting according as the object glass is a lens or a reflector. The object glass must be essentially convex if the telescope is a refractor, and if a reflector, the object mirror must be concave ”

“ Concerning the observing telescope, perhaps Proctor's remarks about a finder for an astronomical telescope may be repeated: "It will be easy for the student to construct one for himself, and will be a useful Fig. 2. exercise in optics." But in case the student may not want to take the trouble, he will be glad to remember that an ordinary pocket telescope or spy-glass may be purchased ready made for a dollar or two, and by removing the erecting lenses a small astronomical telescope may be produced which, as the magnifying power required is small, will answer every purpose of the beginner. ”

“ The luminous rays coming from a distant object are received upon an inclined mirror, which is elevated above the parapet of a fortification, and are reflected downwards to a second, which is placed at a corresponding angle. If necessary, lenses can be interposed, so as to give a magnified view of the distant object that is being examined. By means of such an instrument, the movements of the enemy can be followed without danger, the apparatus being generally of small size, and not attracting notice. Amongst the varieties of this instrument, is one whose use is readily seen by inspecting fig. ”

“ The panoramic sight is a vertical telescope so fitted with an optical system of reflecting prisms and lenses that the gunner can bring into his field any point in a plane perpendicular to the axis of the telescope. ”

“ Fortunately, this study demanded very few auxiliary pieces of apparatus: the telescope has to be directed upon the sun, its motor-clock keeps it constantly pointed upon the same spot, and the observer has to follow, with infinite diligence and patience, the elusive details which the moments of best vision may allow him to glimpse. ”

“ In those of type E the eye-piece has a fixed position and the observer may even occupy a room maintained at uniform temperature, but he must submit to a certain loss of light from one or more reflecting surfaces, and from possible loss of definition from optical imperfection or flexure of the mirror or mirrors. In all these types the longer the telescope and the greater its diameter (or weight) the more massive must be the mounting and the greater the mechanical difficulties both in construction and management. ”

“ Every observer and teacher in astronomy will appreciate the great usefulness of a driving mechanism, which will keep the star in the field during observation. After several years of experimenting we have succeeded in constructing a reliable clock which can easily be attached to our portable telescope mounting. [Pg 7]
The instrument is mounted on strong hardwood tripod fitted with iron shoes. It is attached to the tripod top by a single screw which holds it firmly and allows adjustment in azimuth. ”