OPTICS. 



317 



easily understood. Telescopes are of two kinds 

 the one depending on the principle of refraction, and 

 called the dioptric telescope, the other on the prin- 

 ciple of reflection, and therefore termed the reflecting 

 telescope. 



The great object in constructing a microscope or 

 telescope, is to bring the image of the object as near 

 to the eye as possible, so that it may be viewed 

 under a much greater angle than it could be, were 

 the instrument not interposed between the object and 

 the observer. It is a law in vision, that an object 

 will appear the larger in proportion to the magnitude 

 of the angle formed by any two rays flowing from its 

 opposite extremities ; and the nearer the object is to 

 the eye, the greater will be this angle. Thus, in fig. 

 17, let the arrow A B be placed at a given distance 

 from the eye, the rays from its extremities will con- 

 verge and enter the eye at Q R, they will be refracted 

 by the humours of the eye, as described in a former 

 section, and form an inverted image, M N, on the 

 retina. That a smaller arrow, atC D (nearer the eye), 

 and a still smaller arrow (still nearer), would form a 

 similar image of the same magnitude, will readily be 

 seen by an inspection of the figure. But if an arrow, 

 G H, of the same magnitude as the arrow A B, were 

 held nearer the eye than A B, it would appear 

 larger, because the rays from its extremities form a 

 greater angle. In the diagram, the arrow G H is 

 placed at half the distance of the arrow A B ; and 

 the angle formed by its rays will be twice as large, 

 and consequently the image on the retina twice as 

 large. The image, O P, would appear to be formed 

 by rays from an arrow, T V, double the length of 

 A B, and at the same distance. The apparent 

 length of an object will be diminished one half, by 

 removing it to twice the original distance from the 

 eye ; to one third, at thrice the distance, &c. ; and so 

 will the breadth. Now, a surface, which is only one 

 half as broad, and one half as long as another, is 

 only one fourth the magnitude; if only one third of 

 the length, and one third of the breadth, the magni- 

 tude will be but one ninth, &c.; so that the appa- 

 rent size of objects is inversely as the squares of their 

 distances ; that is, if at sixteen feet distant, an object 

 appear one square foot in magnitude, then at eight 

 feetr it will appear four ; at four feet, sixteen ; and 

 at one foot, as if containing 256 feet. 



By means of a convex lens, a great number of 

 rays from any minute object are united in a sensible 

 point; and as each ray carries with it the image of 

 the point from whence it proceeded, a distinct image 

 will be formed, provided the rays keep their proper 

 relative positions ; and thus it is that a single 

 microscope removes the confusion of rays of an object, 

 when viewed at a short distance from the eye, and 

 thus enables it to examine the object under a greater 

 angle of vision. In the compound microscope, there 

 are essentially two lenses, and its chief superiority 

 over the single microscope is, that by it we obtain a 

 larger field of view. The small object, a b, fig. 18, 

 is placed a little farther from the object-glass, c d, 

 than its principal focus, or focus for parallel rays, so 

 that an image may be formed at g h. This image is 

 viewed by the eyeglass, e f, which is so placed that 

 the eye and the image are in its respective foci on 

 the opposite sides, in consequence of which, the 

 rays enter the eye parallel to each other, after which 

 they are converged upon the retina, and a very large 

 image is formed. 



Fig. 19, represents a very simple form of the re- 

 fracting telescope, in which B represents the eye, 

 m n, the eyeglass (double concave), and o p the ob- 

 ject-glass, (double convex.) There are two pencils 

 of rays proceeding from the arrow x y; those from a; 

 go on diverging till they come to the lens o p, and 



by the refraction of that lens they will be converged 

 in the point E, and so of the rays from y; consequently 

 an image will be formed at E ; but the concave lens, 

 n, being interposed before they meet, prevents the 

 convergence from occurring so soon, and throws the 

 image upon the retina a b, where it is magnified. As 

 the concave lens throws many of the rays beyond the 

 pupil, the field of view is very small. By employing 

 a double convex eyeglass this is remedied, as shown 

 in fig. 20. The object-glass, o p, converges the rays 

 from A and y, and forms an inverted image at m E 

 </, and by interposing the lens g A, this image is seen 

 under a large visual angle, so as to appear of the 

 magnitude C E D. In this telescope the image is 

 inverted, but corrected by interposing other convex 

 lenses between the object and eyeglasses. Here two 

 such lenses are introduced, each of the same focus 

 as the first. 



The great inconvenience attending refracting tele- 

 scopes is their length, and on that account they are 

 not very much used when high powers are required. 

 A reflector of six feet long will magnify as much 

 as a refractor of a hundred feet. In reflectors the 

 concave mirror is substituted for the convex lens. 

 T T, fig. 21, represents the large tube, and 1 1 the 

 small tube of the telescope, at one end of which is 

 D F, a concave mirror, with a hole in the middle at 

 P, the principal focus of which is at I K ; opposite 

 to the hole P is a small mirror L, concave towards the 

 great one ; it is fixed on a strong wire M, and may, 

 by means of a long screw on the outside of the tube, 

 be made to move backwards or forwards. A B is a 

 remote object, from which rays will flow to the great 

 mirror D F. And in order to trace the progress of 

 the reflections and refractions, the upper ones are 

 represented by full lines, the lower ones by dotted 

 lines. Now the rays at C and E, falling upon the 

 mirror at D and F, are reflected, and form an inverted 

 image at m. And they go on towards the reflector 

 L, the rays from different parts of the object crossing 

 one another a little before they reach L. From the 

 mirror L the rays are reflected nearly parallel through 

 P ; there they have to pass the plano-convex lens R, 

 which causes them to converge at a b, and the image 

 is now painted in the small tube near the eye. By 

 means of the lens R, and the two concave mirrors, the 

 image of the object is brought so nigh as at a b, is 

 magnified by the lens S, and will appear as large 

 as c d, that is, the image is seen under the angle 

 cfd, 



This telescope is of the Gregorian form ; but for 

 large instruments the Newtonian reflecting telescope 

 is commonly preferred. A concave mirror, n o, fig. 

 22, is placed in the bottom of a tube, A, D, C, B, which 

 also contains a plane mirror -, at an angle of 45, 

 and fixed upon the arm p q, and in the side of the 

 tube there is an eyeglass, h. Rays from a distant 

 object enter the tube, and fall upon the concave 

 mirror n o, from whence they are reflected, conver- 

 ging upon the surface of the plane mirror y, and 

 thence in the direction q g, being converged in the 

 focus at g, and the image is viewed magnified by the 

 convex eyeglass h. Herschel dispensed with the 

 plane mirror altogether, by inclining the concave 

 one, so that the image might be viewed by the eye- 

 glass alone. See Camera, Kaleidoscope, Phantas- 

 magoria, c. 



Inflection of Light. The direction of the rays of 

 light is changed, as we have seen, in their approach 

 to certain bodies, by reflection and refraction ; and, 

 consequently, we must admit that there is some power 

 in these bodies by which such effects are universally 

 produced. If reflection was produced simply by the 

 impinging of particles of light on hard or elastic- 

 bodies, or if they were in themselves elastic, the 



