Ixxxvi 



INTRODUCTION TO OPTICS. 



attraction of the water acts in the same direction as the course of the 

 ray ; it will not therefore cause a deviation, and the ray will proceed 

 straight on to E ; but if it fall obliquely, as the ray C B, the water will 

 attract it out of its course. L<et us suppose the ray to have reached the 

 surface of a denser medium, and that it is there affected by its attraction. 

 If not counteracted by some other power, this attraction would draw it 

 perpendicularly to the water at B, towards E ; but it is also impelled by 

 its projectile force, which the attraction of the denser medium cannot 

 overcome : the ray, therefore, acted on by both these powers, moves in a 

 direction between them, and instead of pursuing its original course to D, 

 or being implicitly guided by the water to E, proceeds towards F, so that 

 the ray appears bent or broken. C "B^Jig. 19) represents a ray passing 



Fig. 18. 



Fig. 19. 



A 





E 



Fig. 20. 



obliquely from glass into water; glass being the denser medium, the ray 

 will be more strongly attracted by that which it leaves, than by that 

 which it enters. The attraction of the glass would act in the direction 

 AB, while the impulse of projection would carry the ray to F: it moves, 

 therefore, between these directions towards D; so that when a ray 

 passes from a dense into a rare medium, a refraction takes place in 

 the opposite direction to that observed when the ray passes from a rare 

 into a dense medium. The distance at which the denser medium produces 

 its effect upon a ray is so small as to be insensible : the ray appears, 

 therefore, to be refracted only at the point at which it passes from one 

 medium to the other, and passes on in a straight course through each. 



If a shilling (Jig. 20) be placed 

 at the bottom of an empty tea- 

 cup, and the tea- cup at such a 

 distance from the eye that the rim 

 shall hide the shilling, it will 

 become visible by filling the cup 

 with water. In the first instance, 

 the rays reflected by the shilling- 

 are directed higher than the eye, 

 but when the cup is filled with 

 water, they are refracted by its 

 attraction, and bent downwards at 

 quitting it, so as to enter the eye. 

 When the shilling becomes visible 

 by the refraction of the ray, you do not see it in the situation which it really 

 occupies, but an image of "it higher iu the cup; for as objects ,al ways 



