TELESCOPES IKySES. ] 



ASTRONOMY. 



991 



pass through a convex lens. In this case the rays of 

 light coming from the point A, will, as in Fig. 161, con- 

 verge to the point a ; and those from the point B to the 



Fig. 161. 



focus b ; and in the same manner, all the other points of 

 the object A B will be represented by opposite points in 

 the image formed by the lens. It thus follows that the 

 image is reversed ; and if the eye be placed at the focus 

 a b, the object A B will appear turned upside down ; but 

 in all other respects, a perfect picture of the object A B 

 will be perceived. This effect may likewise be seen by 

 I placing a candle, A (Fig. 162), in a dark chamber, at a 



Fig. 162. 



certain distance from the mounted lens B, the light 

 from the candle passing through the optical axis of the 

 lens, to the opposite wall, C, which is at the proper dis- 

 tance from the lens to receive a well-defined image 

 of the flame of the candle. A reversed image of the 

 candle may here be perceived, as well as of that 

 portion of the candle which is illumined by the flame. 

 The well-known effect of convex glasses in mag- 

 nifying small objects, will be seen from the fol- 

 lowing diagram (Fig. 163). The object A B being 

 Fig. 163. 



vision is different for almost every person ; and, in tho 

 second place, when we look through magnifying glasses, 

 almost every guide which serves to regulate the judg- 

 ment, on the distance and size of the object 

 looked at, is removed. 



TELESCOPES. Every convex glass, as wa 

 have seen, produces an image of the object 

 from which it receives rays of light ; and 

 by a combination of such glasses, viz., by 

 B examining the image formed by one lens 

 (as if it were a real object) by means of 

 another, and thereby magnifying the image a." ex- 

 plained in the last paragraph, distant objects can 

 be seen with a distinctness unapproachable with the 

 naked eye. The most simple and most common de- 

 scription of the telescope, used by the astronomer, is 

 that formed by two convex lenses, L and L' (Fig. 101) ; 

 the first of which, from being turned in the direction of 

 the object, is termed the object-glass ; the second the eye- 

 piece. In this, the rays of the object A B, passing 

 through the convex lens L, form an image a b 

 in the focus of the lens. The second lens serves 

 to magnify the reversed image 6 a in the 

 same manner as if it were a tangible object. 

 The image 6 a is not always at the same dis- 

 tance from the object-glass, but varies more 

 or less according to the distance of the object ; 

 and when the latter is so far removed that the 

 rays which fall on the object-glass may be 

 considered as parallel which is the case with 

 all celestial objects the image 6 a falls in 

 the focus of the lens. As the object should be 

 seen with the requisite distinctness at b' a', 

 which is the distance of distinct vision, and as 

 the latter varies almost with every person, 

 it is necessary that the eyepiece should 

 be drawn in or out from the image 6 a, 



Fig. 164. 



placed between the lens and its focus E, the rays of 

 ight proceeding from the point A do not lose all their 

 divergence, but appear to come from a more distant 

 joint a, formed by the prolongation of the line O A ; and 

 n the same manner the point B seems to be situated at 

 i, formed by the prolongation of the line B O. To the 

 eye situated at the other side of the lens, therefore, it 

 will appear as if the object A B were replaced by the 

 image a 6, and the latter object will appear more or less 

 distant from the eye, according as the object A B is 

 learer or more removed from the focus of the lens. The 

 ens can be so shifted in respect to the object, that the 

 mage a b will appear distinct and well defined, as well as 

 magnified. We can readily perceive that the image a b 

 s greater and more distant when the focal distance is 

 mailer, and, in consequence, that a lens magnifies so 

 much the more as its focal distance is less. The magni- 

 ying power of lenses is somewhat illusory, and nearly 

 very one judges differently in estimating the magnified 

 \7.<: i if 1 1 ic nliji-rt under examination. This may arise from 

 wo causes in the first place, the distance of distinct 



in order to give perfect definition. It will be seen 

 from the diagram, that the telescope does not, like 

 the microscope or magnifying -glass, increase the 

 natural size of the objects viewed ; for the image 6' a' is 

 much smaller than the object A B, which is at a great 

 distance from the object-glass : it only tends to increase 

 the apparent size of an object when viewed from a dis- 

 tance. 



To compare the size of a distant object when viewed 

 with natural and telescopic sight, we have only to com- 

 pare the angles A o B (Fig. 1(54), or a o b, which is the 

 angle subtended by the object to the unassisted vision, 

 and the angle a' o' b', which is the angle subtended by the 

 image to the eye of the observer. The proportion be- 

 tween the relative sizes of the object A B, and the image 

 a.' b', is consequently the same as that between the angles 

 a ob and a o' b ; and this is what is termed the magnify- 

 ing power of the telescope. The angle a'o'b' and a o 6 

 being always small, they may be regarded as in the 

 inverse proportion of the two points o and o' from the 

 image a b ; or, in other words, that as these distances 

 may be regarded as the focal lengths of the object and 

 eye-glasses, that the magnifying power of the telescope is 

 in the same proportion. This instrument allows a large 

 field of view (or the circular space which the eye would 

 take in without the assistance of any telescope), for this 

 depends on the dimensions of the instrument at a 6, or 

 the common focus of both lenses. Equally important 

 with the perfect definition of the object, is, as already 

 stated, the brightnest with which it appears when under 

 examination. If a ray of light pass from any point, M 

 Fig. 16">), of an object, it will fall on the whole surface 

 of the object-glass, and thence through the same point, 



