Telescope (Gr.τήλε , far, and σκοπέιν, to view), an instrument for aiding the eye in viewing distant objects. The general construction of the telescope is based upon the property possessed by a convex lens or concave mirror of converging to a focus the rays of light falling upon it from any object, and of forming at that focus an image of the ob-ject itself. This image may be rendered visible, as in the camera obscura, by interposing at the focus a white screen, a plate of ground glass, or a cloud of light smoke within which the image will appear suspended'. But if the rays be allowed to proceed without interruption, and the eye be placed in the axis of the lens or mirror and at the proper distance from the focus, the image will be seen more distinctly than before; and if the focus be nearer to the eye than to the lens, the apparent dimensions of the image will be greater than the apparent dimensions of the object itself. This is the simplest, though not the common form of the telescope.
Usually a second lens, of shorter focus than the first, is introduced near the image, the effect of which is to increase still further the apparent magnitude of the object; and thus is constituted the ordinary telescope, which in its elementary construction consists of an "object glass" or "object mirror," of as large dimensions as practicable, and an "eye lens," which enables the eye to receive the image under the greatest practicable angle. In tig. 1, M is the object glass and N the eye lens. The inverted image b a of a distant object A B is formed between the eye lens and its principal focus, and the eye lens then gives a magnified image of it, b' a'. The object glass is always necessarily convex, and the mirror concave, but the eye glass may be either; if convex, it is placed at the proper distance beyond the focus, and, the rays having crossed, the image then appears inverted; if concave, as in the common opera glass, it is placed within the focus, and objects appear in their natural position. The magnifying power of the instrument is measured by dividing the focal distance of the object glass by that of the eye piece; the illuminating power depends mainly on the size of the object glass.
In the terrestrial telescope, commonly called spy glass, the image is produced in its natural position. To effect this two additional lenses, O and P, fig. 2, called condensing glasses, are introduced between the real image and the eye lens. The object A B produces an inverted and smaller image at b a. The lens O being at the distance of its principal focal length from b a, the rays which fall on P will be parallel, and the image a' b' in the principal focus of P will be erect, as will also be the magnified image a" b". - It is believed by many authorities that the theory of both the telescope and the microscope was known to Roger Bacon, and the telescope is said to have been used by Digges before the 17th century; but the first really definite accounts of the invention date from the latter part of the year 1608. Magnifying lenses had long been known, and even the compound microscope had been invented by the Jansens nearly 20 years before this date; a discovery which has somewhat embarrassed the study of the question before us from confusion of the by no means explicit terms in which both instruments are described.
But it is now generally conceded that the honor of making the first telescope belongs to one of two individuals, Hans Lippersheim, a spectacle maker in Mid-delburg, and Jacob Adriansz, called also Me-tius, a native of Alkmaar. Lippersheim, on Oct. 22, 1G08, presented to his government three instruments with which " one could see things at a distance," applying at the same something in their very nature or use, or some other circumstance of equivalent force, distinctly implies that they are to be left at some other place. And it may happen from the cumbrousness of the articles, or other circumstances, that it is obviously reasonable and just for the deliverer to ascertain from the receiver, long enough beforehand, where they shall be delivered; and then he will be held to this as a legal obligation. If the receiver refuses or neglects to appoint a place, or purposely avoids receiving notice of a place, the deliverer may appoint any place with a reasonable regard to the convenience of the other party, and there deliver the articles.
If no expressions used by the parties and nothing in the nature of the goods or the circumstances of the case control the presumption, then the place where the promise is made is the place where it should be performed; and no action can be maintained upon such a promise unless the plaintiff can show a demand at the proper place and time, or a readiness to receive, and notice equivalent to a demand, or else that the demand would have been nugatory because the defendant could not have complied with it. If by the terms of the contract specific articles are to be delivered at a certain time and place in payment of an existing debt, this contract is fully discharged and the debt is paid by a complete and legal tender of the articles at the time and place, although the promisee was not there to receive them; and no action can be thereafter maintained on the contract. But the property in the goods has passed to the creditor, and he may retain them as his own, or take them elsewhere; or he may demand them, and if they are refused bring an action for them as his own. TENDON, the fibrous cord or expansion by which a muscle is connected with the surface of bone.
Tendons are composed of parallel bundles of white, inelastic, inextensible, fibrous tissue, the spaces between which are occupied by thin layers of loose areolar tissue, with a small proportion of elastic fibres, sufficient to allow a slight gliding motion of the different tendinous bundles upon each other. As a whole, however, the tendon is both inextensible and inelastic, and thus conveys at once the movement imparted by the muscular contraction to the bone into which it is inserted. The typical form of a tendon is that of a long, flexible, cylindrical cord, like those at the lower part of the forearm, for the flexion of the wrist and fingers. Others are more or less spread out into a ribbon-like form, like that of the sterno-mastoid muscle at the upper extremity of the sternum; while others are expanded into a broad and thin sheet or aponeurosis, like the tendinous expansions of the latissimus dorsi, or of the muscles on the anterior part of the abdomen. The long and cord-like tendons often run in narrow grooves of bone, in which they are confined by fibrous sheets passing over them from edge to edge.
Their movement is sometimes facilitated by the existence of closed sacs or bursae, situated between them and the bony surfaces over which they pass, and filled with a glairy lubricating fluid. Sometimes, as in the case of the tendon of the superior oblique muscle of the eyeball, they pass through a pulley-like loop or fibrous ring, and then return in an oblique direction to be inserted somewhere between the loop and their point of origin. Sometimes they have developed within them at certain points, where crossing articulations, small bones termed "sesamoid bones," the inner surface of which takes part in the formation of the joint. The patella, or knee pan, is regarded as an unusually large sesamoid bone, developed in the tendon of the great extensor muscle on the front part of the thigh. - Owing to their strong fibrous texture and inextensible quality, the tendons, when contracted or bound down by unnatural adhesions, are liable to produce or perpetuate deformities, particularly in the neck and the extremities. They require, under these circumstances, to be divided by a subcutaneous incision, releasing the contracted parts without bringing the air into contact with the wounded surface.
This practice, known as "tenotomy," is largely resorted to in cases of wry neck, club foot, and many similar deformities.
Fig. 1. - Astronomical Telescope.
Fig. 2. - Terrestrial Telescope.