OPTICS 



become more indistinct, and the colours 

 more faint, and when the hole reaches a 

 certain size the red light from the roses 

 will fall upon the same parts of the card 

 as the green light from the leaves, and 

 the card will appear of its original white- 

 ness. The same appearances will be 

 seen in whatever position we hold the 

 box in relation to the rose bush. The 

 reason will be explained in a future 

 Chapter, why the white light of the sun 

 becomes red when thrown off from the 

 rose, and green when thrown off from 

 the leaves. 



4. Light consists of separate parts 

 independent of each other. If we admit 

 light through an opening into a dark 

 room, we may, by interposing a piece of 

 card, stop a small portion of it, and allow 

 the rest of it to pass ; or, if we stop nearly 

 the whole of it, and allow only a very 

 small portion of it to pass, the part 

 which, in both these cases, is allowed 

 to pass, is not affected in any way by 

 its separation from the part which is 

 stopped. 



The smallest part of light which we 

 can thus stop, or allow to pass, is called 

 a ray of light. 



5. Rays of light proceed in straight 

 lines. This property may be demon- 

 strated to the eye, by causing light to 

 pass through small holes into a dark 

 room filled with smoke or dust. It is 

 proved also by the fact, that bodies can- 

 not be seen through bent tubes ; and it 

 may be inferred from the form of the 

 shadows of bodies. When there is any 

 power on one side of the ray which is 

 not on the other side, it may then de- 

 viate from its motion in a straight line, 

 and may even be made to move in a 

 curve line. 



6. Light moves with prodigious velo- 

 city, and that of the planets travels at 

 the rate of 195,000 miles in a second of 

 time. If two observers are placed at 

 the distance of 70 or 80 miles, and if one 

 of them strikes a light at a known in- 

 stant of time, the light will travel to the 

 other observer in such a small portion 

 of time that it cannot be measured by 

 the nicest time-keepers. Astronomers, 

 however, have proved, by observing the 

 eclipses of Jupiter's Satellites when that 

 planet is nearest and farthest from the 

 earth, that light travels from the sun to 

 the earth in seven minutes. Hence, it 

 will move from the one pole of the globe 

 to the other in the 24th part of a second, 

 a velocity which surpasses all com- 

 prehension. 



CHAPTER II. DIOPTRICS. 



Refraction of Light Law of Refraction 

 Refractive Powers Table of Re 

 fractive Powers. 



ALTHOUGH a ray of light will always 

 move in the same straight line when it 

 is not interrupted, yet every person must 

 have observed that when light falls 

 upon a drop of water, or apiece of glass, 

 or a bottle containing any fluid which 

 allows the light to pass, it does not 

 reach the eye, or illuminate a piece of 

 paper, placed behind those bodies in the 

 same manner as before they were put in 

 its way. This obviously arises from the 

 direction of the light being changed 

 by some power which resides in the 

 bodies. The branch of optics which ex- 

 plains the law or rule according to 

 which the direction of light is thus 

 changed, and the results dependent on 

 this law, is called DIOPTRICS, from two 

 Greek words, one of which signifies 

 through, and the other to see, because 

 the bodies which produce this change 

 are those through which we can see, or 

 through which light passes. 



In order to understand how this 

 change is produced, let M N O P (fg. 1 .) 



Fig. 1. 



be a vessel, in one of the sides of which, 

 N O, there is a small hole at N. If we 

 place a lighted candle within two or 

 three feet of it, so that its flame may be 

 at A, a ray of light, A N, proceeding 

 from it, will pass through the hole N, 

 and go on in a straight line A N C, till 

 it reaches the bottom of the vessel at C, 

 where it will form a small circle of light. 

 Having put a mark at the point C, pour 

 water into the vessel till it rise to the 

 height s r, and you will see that the 

 round spot which was formerly at C is 

 now at D ; that is, the ray A N, which, 

 when the vessel was empty, went straight 

 on to C, has been bent at the point B, 

 where it falls on the water, into the line 

 B D. If we mix a little soap with the 



