ASTRONOMY. 



[TELESCOPES LENSES. 



The distinctness of vision, as every one is aware, depends 

 likewise on the distance of the eye from the object ; and 

 this distance U variable for different persons. Even the 

 one eye of the name in.livi.hi.il may have a longer range 

 of vision than the other. Every one knows, also, that if 

 an object be held at a greater or less distance from the 

 ye than that which U customary or natural, in either 

 case it becomes indistinct. 



In regard to the brightness of any object, according as 

 it is more or less removed from the eye, the following 

 considerations are to be taken into account. From any 

 bright object as M, rays of light are sent in all directions ; 

 and from the point m, as from any other point on its 

 surface, these rays are sent before it, as represented in 

 ISO. Fig. 150. Of all 



the rays of light, 

 the eye only receives 

 those which pene- 

 trate through the 

 pupil a It. If the 

 eyu now approach 

 the object M, so 

 that its distance is 

 one-half of what it 

 was in the previous 

 case, and its position 

 a' It', the diameter of the cone of light will be only one- 

 half of what it was in the previous case, and consequently 

 the area of the section in that part, is only one-quarter 

 of that at the base a b, where it was equal in area to the 

 surface of the pupil of the eye. Hence it follows that 

 tin- opening of the pupil at a' '/ will take in a greater 

 number, by four times, of rays, than when it was situated 

 at o 6, so that the light received from the point m will be 

 four times as much in the latter case, as in the former. 

 At the same time, however, as the eye approaches the 

 object M, a greater portion of its surface is taken 

 in, and the position m becomes four times as great 

 The bright light is thus spread over a greater sur- 

 face, and in a like proportion ; and it follows that 

 the brightness of the surface is unchangeable. If 

 it happen that this is not the case in nature, it 

 must be remembered that the interposition of the 

 air causes a loss of light, and even a change in the 

 colour of objects ; for distant mountains appear 

 blue, as likewise does the sky being the effect 

 when viewed through great depths of the atmosphere. 

 Without this, a white object would appear of equal 

 brightness whether it were seen close at hand or at a 

 distance. 



LENHBS.* Many solid bodies, and a greater 

 portion of liquid ones, are transparent; but, in 

 order that the former become perfectly so, it is 

 necessary that their surfaces should be highly 

 polished, which condition is fulfilled, in liquids, by 

 the natural effect of gravity. In the grinding and 

 polishing of glass and crystal, or even reflecting 

 metals, into various shapes, it was early discovered 

 that various effects were produced ; but none more extra- 

 ordinary than those which took place in the apparent mag- 

 nitude of objects when viewed through these artificial 

 media. The fundamental law on which all those changes 

 are based is, that when a luminous ray passes obliquely 

 from one transparent medium into another, it departs 

 from its primitive direction, ami undergoes a refraction. 

 On this law the theory of lenses U based. 



There are two different species of lenses, the surfaces 

 being, in both cases, spherical, or one of the faces may 



ill. Pig. us. Fig. iss. k Pl ane - The first are 

 those in which the surface is 

 convex or converging, and 

 the other in which it is con- 

 anil diverging. The 

 rgiX glasses are those 

 are doubly convex 

 (Fig. 151), the form being 



* Th (MMfWtlM of minr of Itir following trrantrmrau bu ilmd? 

 i. r i .'.!; IMOTUBI !-.. i. lh. on on 



I Ml ipKUIIjr m rrfrrm* to ulronoiiij, to which Uie 



ilar (whence they derive their general name of 

 lenses) 2 those which are plano-convex (Fi,-. 1 .">.'), and 

 3" concave-convex (Fig. 153), the c.m\. _, in 



this case, more considerable than the con.-avity and 

 which are likewise known by the name of menis- 

 consequence of the form of 

 their section. The converg- 

 ing glasses are those which 

 are doubly concave, as in 

 Fig. 154 ; plano-concave, or 

 convex-concave, the con- 

 cavity being more consider- 

 able than the convexity, 

 and likewise known as menisci. The railii of lenses are 

 the radii of the spheres of which their surfaces are the 

 segments. 



If we expose a convex glass to the rays proceeding 

 from the sun, they will be converged to a point, A (Fig. 

 157); and if we remove the lens more or less away from 

 the surface on which the converging rays are received, 



Fig. 157. 



PUr. 1M. Fig. us. rig. 156. 



n c 



this bright spot (A) will be of greater or smaller extent. 

 To that point where it is of the smallest dimensions, the 

 name of focus has been given, and its distance from the 

 nearest surface of the lens i.< called the focal distance. If 

 we turn the lens round, the same effect is produced ; 

 and if both the surfaces have the same radius, the dis- 

 tance will be alike in both cases, though it will vary but 

 rig. 158. 



very slightly should they be different even in the case of 

 a meniscus lens. 



The line a A (Fig 158) is termed the axis of the lens, 

 and the point o its optical centre. In the above case the 

 He. 159. 



rays are supposed to pass parallel to the optical axis ; 

 but if they proceed from a luminous [xjint, A, a short 

 distance from the axis (Fig. 159), or rather, if the parallel 

 rays make a small angle with the optical axis, the manner 

 in which they converge to a focus, o, is the same. In 

 Until cases they pass through the optical centre o of the 

 lens (Fig. ICO). In concave lenses, the parallel rays of light 



Fig. 160. 



pass from an object are rendered divergent; or if 

 already divergent, are rendered more so by their inter- 

 position, as in Fig. 166, p. W2. 



Supposing that, instead of proree.linu' from a point, \, 

 the rays of light from a luminous object, A B (Kii; li'il), 



