OPTICS. 



13 



rays diverging from a point, as shewn in 

 fp. 9. This motion of the conjugate foci 

 is excellently illustrated by the following 

 experiment given by Dr. Smith. 



Having determined the focal distance 

 E F, (fig. 15,) of a convex lens of glass 

 E, and fixed it in a frame on a stand of 

 wood C E D, placed vertically on a long 

 table or floor A B, draw a line A B 

 perpendicular to the frame, or parallel 

 to the axis of the lens, through the 

 point C, and on this line lay down the 

 focal distance of the lens from C to F, 

 and set the same distance from F to I, 

 from I to II, II to III, &c., and also 

 on the other side of C, from C to/, 

 from /to 1, from 1 to 2, 2 to 3, Sec. 

 Let A A i &c. of the focal distance E F 

 be next set from F towards I, and also 

 from /towards 1, and affix the numbers 

 4 ^ i to the points of division. When 

 this is done, darken the room, and set a 

 candle at Q over the mark II, the rays 

 refracted by the lens will converge at q, 

 and form an inverted image of the 

 candle upon a paper screen G H, placed 

 at the opposite mark |. If the candle 

 is removed to III, the inverted image 

 will be seen formed at 3, by advancing 

 the paper to , and if the candle is 

 pushed farther in to I, the distinct image 

 will be seen formed at 1 , by withdrawing 

 the paper to that point. The effect will 

 be exactly the same if the candle and 

 the paper be made to change places. 

 Hence it will be found by direct experi- 

 ment, that / q varies reciprocally as 

 F Q, that is, it increases in the sams 

 proportion as F Q diminishes, and dimi- 

 nishes in the same proportion as F Q in- 

 creases. If Q is brought forward to F, 

 no distinct image of the candle will be 

 formed at any distance, but the light 

 will be refracted into a parallel beam, 

 of the same diameter as the lens at all 

 distances from it. 



If the image n m (fig. 14.) is received 

 upon ground glass, or upon transparent 

 paper, or upon a plate of glass upon 

 which a layer of skimmed milk has been 

 allowed to dry, and if we place the eye 

 behind it, we shall see the inverted 

 image with as much distinctness as 

 before, provided the eye is distant six 

 inches, the distance at which we view 

 all other near objects. If when the eye 

 is in this position, we take away the 

 glass or paper on which the image is 

 formed, we shall see the image in the air, 

 as it were, as distinctly as before. The 

 reason of this is, that all the rays which 

 are refracted to foci at n t m, &c. cross 



Fix. 15. 



