OPTICS. 



15 



see it distinctly. This is effected by 

 placing close to the eye a convex lens, 

 and holding the object in its focus ; for 

 we have already seen that when rays di- 

 verge from/, the focus of a lens L L, 

 Jig. 7, the refracted rays L R, L R are 

 parallel. By placing the object a little 

 nearer than/, the rays which flow from 

 it may receive that precise degree of 

 diveigency which they have when the 

 object is placed six inches from the eye, 

 the nearest distance at which we see 

 minute objects distinctly. If the distance 

 C/is one inch, the object at/ will have 

 its apparent magnitude six times greater 

 than when it is seen at the distance of 

 six inches without the lens. It is, there- 

 fore, said to be magnified six times by 

 the lens L L. This lens is called a single 

 microscope, and the magnifying power of 



Fig. 



1 



such microscopes may be always found 

 by dividing six inches by the focal dis- 

 tance of the lens. A lens, for example, 

 the tenth of an inch in focal length, will 

 magnify 60 times ; and one the hun- 

 dredth of an inch, 600 times. 



To the telescope with one lens, which 

 magnified 10 times, when the image 

 mn (fig s - 14 an d 16) was seen by the 

 naked eye placed six inches behind it, 

 we may now give additional magnifying 

 power, by bringing the eye O within an 

 inch of the image, that is, by viewing 

 the image nm with a lens II, whose 

 focal distance is one inch. This lens 

 will magnify the image six times ; but, 

 as the image had been previously mag- 

 nified ten times, by the lens L L, the 

 magnifying effect of the two lenses will 

 be 10x6, or 60 times. This instrument 

 16. 



is the astronomical telescope, by which 

 objects are seen inverted, and the mag- 

 nifying power of which is always equal 

 to the focal length of the object glass 

 L L, or the lens next the object, divided 

 by the focal length of the eye glass 1 1, 

 or the lens next the eye. 



The principle, therefore, of the tele- 

 scope, is simply this : the object glass 

 forms, in its focus, a distinct image, or 

 picture of the object, which, though 

 very much smaller than the object, is 

 yet seen under a much greater angle, or 



magnified ; and this image, so magnified, 

 is seen under a still greater angle, or 

 still farther magnified by the eye glass, 

 which enables the eye to see it distinctly 

 at a less distance than six inches. 



The process of magnifying objects by 

 the single microscope* has been already 

 explained ; but, when a very high mag- 

 nifying power is required, it is necessary 

 to use two lenses, as in the astronomical 

 telescope. The object M N (Jig. 1 7.) 

 is placed a little farther from the lens 

 L L than its principal focus, and an in- 



Fig. 17. 



verted image of it is formed at nm. 

 This image, beins; in the principal focus 

 of another lens //, the rays which pro- 

 ceed from it will be refracted into pa- 

 rallel directions, and thus afford distinct 

 vision of it to the eye at E : the lens 

 L L is called the object glass, and / / 

 the eye glass, and the instrument is 

 called a compound microscope. The ob- 

 ject M N is first magnified by the object 

 glass L L ia the proportion of L m to 



L M, and this magnified image is again 

 magnified by the eye glass / /, in the 

 proportion of In to "six inches. Hence, 

 if the focal length of L L is half an 

 inch, L n six inches, and the focal length 

 of// one inch, the magnifying power 



will be 1 x ~ =72 times, OB 12 times by 

 L L, and again 6 times by //. 



* From two Greek words, 

 things. 



signifying to sec small 



