TEL 



157 



TEL 



Teleosaur, regrets his inability to state whether either ol 

 the articular extremities of the centrum were convex, or 

 which of them. The Whitby Teleosaur decides this ques- 

 tion, and shows that both articular extremities of the ver- 

 tebrae are slightly concave in the cervical as in the rest of 

 the vertebral series. The atlas in the Teleosaur corre- 

 sponds essentially with that of the crocodiles, as is shown 

 by the three main component parts of this bone, from a 

 Whitby Teleosaur in Lord Enniskillen's collection. The 

 body or centrum is a transverse quadrilateral piece, 

 smooth and convex below, narrowing like an inverted 

 wedge above, with articular facets, viz. : a concavity in 

 front for the occipital condyle ; a flat rougher surface on 

 each side of the upper part 1'or the attachment of the neura- 

 pophyses ; a posterior facet for the anterior part of the de- 

 tached odontoid element of the axis ; and the small sur- 

 face on each lateral posterior and inferior angle, for the 

 atlantal ribs. The neurapophyses are pyramidal pro- 

 cesses, with their apices curved towards each other ; they 

 are relatively smaller in proportion to the centrum than 

 in the crocodile. The general anterior concavity for the 

 reception of the occipital tubercle is formed at its cir- 

 cumference by the centrum and neurapophyses of the 

 atlas, and at its middle by the anterior detached odontoid 

 epiphysis of the axis, which is here evidently the analogue 

 of the so-called atlas in the Ichthyosaurus, the true body 

 of the atlas in the Teleosaur representing the first in- 

 verted wedge-shaped bone in the Ichthyosaur. The 

 spine of the atlas is a hu-ge oblong piece articulated with 

 the neurapophyses of the atlas, and partly overlapping 

 those of the axis.' 



The Professor then describes in detail the cervical ver- 

 tebrae, from which it appears that the same mechanism 

 for fixing and strengthening the neck, as is found in ex- 

 isting species, exists in the ancient marine crocodiles ; 

 the vertebra? of the dorsal region, with 16 pairs of ribs, a 

 greater number than occurs in any existing Crocodilian ; 

 a posterior dorsal or lumbar vertebra, which faintly in- 

 dicates one of the most striking characters of the ver- 

 tebrae of STREPTOSPONDVLUS ; and the caudal vertebrae, 

 which progressively diminish in every diameter, save 

 length, from the middle to near the end of the tail, the 

 terminal vertebrae being shorter than the rest. The ster- 

 num and sternal ribs, he remarks, closely agree with the 

 ordinary Crocodilian type. He had not seen a specimen 

 of the abdominal sternal ribs. 



Professor Owen then notices the structure of the Pecto- 

 ral and Pelvic Extremities, as compared with those of 

 existing Crocodiles, and the Dermal armour, the bony 

 dermal scutes of which are regularly disposed in the 

 Teleosaur, as in them ; but the scutes of Teleosaurus Chap- 

 manni, he observes, differ as much from those of the ex- 

 isting Gavials and Crocodiles, as those of Teleosaurus 

 Cadom#ntit do. The following are the species recorded 

 by the Professor: Teleosaurus Chnpmanni ; Teleosaurus 

 Cadomensis ; Teleosaurus Cadomensts, var. ; and Teleosau- 

 rus asthenodeirus, Owen. [STENEOSAURUS ; CROCODILE, 

 vol. viii., p. 168.] 



TELESCOPE (from the Greek telescopos, rijXtwoTOc, 

 ' far-seeing'), an optical instrument consisting of a tube 

 which contains a system of glass lenses having all their 

 centres in one common axis, or a tube containing a me- 

 tallic speculum in combination with such lenses: by 

 either kind of instrument distant objects are caused to 

 appear magnified, and more distinct than when viewed by 

 the naked eye. Those which are constructed with glass 

 lenses only are called dioptric, or refracting, and the others 

 catoptric, or reflecting telescopes. In the former kind the 

 rays in the pencils of light which come from every part 

 of the object viewed are, by the first lens on which they 

 are incident, made to converge so as to form an image at 

 the focus of the lens. In some eases the rays in each 

 pencil are intercepted by a second lens, and, by its refrac- 

 tive power, are made to enter the eye in parallel direc- 

 tions : in other cases, the rays, after having crossed each 

 other at the place where the image is formed, fall in a 

 divergent state upon a second lens, and by it are refracted 

 no as to emerge from it in parallel directions. Frequently 

 however the parallelism of the rays is effected by two or 

 more lenses in addition to that, called the object-glass, by 

 which the image was formed. In reflecting telescopes an 

 image is formed by the reflection of the rays in the pencils 

 of light coining from the object, after having impinged 



upon the concave surface of the speculum : in some cases 

 this image is viewed through one glass lens or more, but 

 frequently the rays, before or after forming the image, are 

 reflected from a second mirror, and are subsequently trans- 

 mitted to the eye through lenses. 



By these instruments objects even in the remotest depths 

 of space are rendered accessible to human vision ; and 

 terrestrial objects faintly visible in the distance are brought, 

 as it were, close to the eye. In the hands of astronomers 

 they were the means, almost immediately on being in- 

 vented, of making more discoveries in the heavens than 

 had been made during 5000 years previously; they form a 

 valuable addition to the instruments employed by the 

 mariner and the surveyor, and they will ever constitute 

 the most agreeable companion of the traveller, by ena- 

 bling him to distinguish, in every direction from him, 

 objects which it might be difficult or impossible for him 

 to approach. 



In exhibiting the principles on which a telescope is 

 constructed, it will be proper to commence with an ex- 

 planation of the means by which the image of an object 

 is formed at the focus of a lens or of a reflecting mirror. 

 With respect to a lens, if it be of the kind called convex 

 [LENS], the rays in the pencils of light which proceed 

 from every part of an object, as APB, in passing through 

 the lens, supposing the latter to have a proper degree of 

 curvature, are made to converge by the refracting power 



of the glass at points, as a, F, and b, and the assemblage of 

 such points constitutes an image of the object : if a screen 

 were placed at F perpendicularly to the axis PF, the 

 object would be represented on it, in an inverted position. 



If the lens were of a concave form, the rays in the 

 several pencils, after passing through it, would be made to 

 diverge from one another, and consequently no image 

 could be formed : yet if the directions of the rays, after 

 refraction, were produced backwards, they would unite 

 between the lens and the object, in points corresponding 

 to those which constitute the image formed by the convex 

 lens. 



If the rays in the pencils of light proceeding from differ- 

 ent points, A, P, B, in an object are reflected from the sur- 

 face of a concave mirror, supposing the latter to have a 



certain degree of curvature, those rays will unite in as 

 many points, a, F, and b, and form an image of the object. 

 If a screen were placed at F the object would be repre- 

 sented on it, in an inverted position. The rays in each 

 pencil reflected from the surface of a convex mirror are 

 made to diverge from each other; and in that case no 

 image is formed. 



Now, if the object AB be so remote that, in each pencil, 

 the rays incident upon a lens may be considered as paral- 

 lel to one another, the point F is called the principal 

 locus ; and in the article LENS (p. 421, col. 2) there will 

 be found a collection of formulae for the reciprocals of the 

 focal lengths of lenses of all the different kinds ; it being 

 understood that the diameter of the lens is small, which is 

 generally the case with telescopes, and that the light is 

 homogeneous. But, since all light is not of one kind, and 

 a lens acts like a prism in causing, in each pencil, the rays 

 of the differently coloured light to diverge from one 

 another : it follows that each of the coloured lights will 

 form its own image at its proper focus; and the image 

 formed by light of one kind being seen by the eye along 

 with the images formed by light of the other kinds, the 

 representation of an object when formed by a single lens 



