RECEEATIVE SCIENCE. 



KEci; I:\TIVI: SCIENCE. vi 



FRACTION, LKNSKS, AND MA< . 1'OWER. 



.FRACTION from re (back) and/ranjro (to bond), the bonding 

 - k of tho ray of light ia a property of light which tuu tempted 

 _ny a youth to plungo into water that appeared shallower 

 . .11 it roully was, because a place BIZ feet deep would eem to bo 

 ily a depth of fonr foot and a half ; and upon thia fact may be 

 founded tlui qurstion, \Vliy does the water appear to be shallower 

 tliiui it really is ? Tho answer ia beat given experimentally. 

 i a ].-ii.-il of light falls upon tho aurfaoe of wat. : 

 irulur direction, nearly every portion passes into, and is 

 ittcd by tho latter. If the raya fall obliquely upon th-- 

 iter some of them are reflected, whilst that portion 

 iters tho water does not pursue a straight line, bat ia bent in 



body ia that which uu the hiffteat refracting power; aad taia 

 being understood, the abore general sssnrHoo fe mMj proved 

 by allowing a sunbeam to pau throagh a hole in a shatter, ao 

 that it may fall upon the bottom, c, of aa empty glass flak, 

 globe, A B, or other convenient basin. (Pig 



Tho place, c, where the my of light strikes the gl*H, ahoald 

 now be marked by laying a pfeo* of bright ailver there, and 

 when the globe ia filled with water, without moving ft from it* 

 poaition, the ray no longer fall- upon the .pot where UM silver 

 was placed, bat at D; benoe "the angle* of incidence aad re- 

 fraction are in the aame plane perpendicular to the refraetiag 

 aurfaco," oorreaponding in thia eiperimcnt with the hatter-line 

 throagh which the beam of light pavwea. If the water in th globe 

 ia made Blightly opalescent by the addition of a few drop* of 

 milk, tho courw of tho refracted ray ia very nicely marked oat. 



XI 



proc 

 watc 



i= V,., 





direction towards or nearer to tho perpendicular ; and it is 

 thia bending of the ray which is called refraction. 



Thus the ray A B (Fig. 1) enters tho refracting surface R B, and 

 ceeds to c in the same direction, whilst D B, entering tho 

 ,ter obliquely, instead of proceeding in the same direction B E, 

 is bent in the direction B o, which makes a less angle with the 

 perpendicular, B c, than if it had proceeded to K. 



It is therefore asserted, with certain exceptions, that a ray 

 of light in passing from a raro into a denser medium ia bent 

 towards the perpendicular, and the contrary when the pencil 

 of light emerges from a dense into a rarer medium. It must 

 be understood that the density or rarity of the body referred 

 is not that ot specific gravity. Optically considered, ono 

 ly may be denser than another, whilst, physically, it is veally 

 ighter ; thus, oil of turpentine is lighter than water, bnt has a 

 higher refracting power than the hitter, BO that a ray of light 

 passing from turpentine into water is refracted from tho perpen- 

 dicular, and in passing from water into turpentine it ia bent 

 towards the perpendicular. In optical language the densest 



98-N. 



and when allowed to fall upon a piece of looking-glass it ia aecn to 

 bo reflected through the water, according to the laws of reflection, 

 and emerging from the denser medium, water, into the rarer one, 

 air, the bending of the ray from the perpendicular is Been : thus 

 both *efraction and reflection are illustrated in these experiment*. 

 The sunbeam, A B (Pig. 3), falling on the surface of the water, 

 where a plummet line, p, ia suspended, ia bent or refracted in the 

 direction D c, or towards the perpendicular ; the ray, B c, falling 

 on the looking-glass, L, is reflected at an equal angle throagh 

 the water to D ; here, on emerging into the air, it ia refracted 

 from the perpendicular or plummet bine, L, in the direction D K ; 

 hence " the sine of the angle of incidence, divided by the sine of 

 tho angle of refraction, is a constant quantity," and ia called the 

 index of refraction. The fine ia a perpendicular line drawn from 

 the extremity of an arc to the diameter of a circle. The law of 

 sines published by Descartes, and known as Descartes' law, waa 

 discovered by Willebrord Snell, a Dutch mathematician, about 



At the back of a white plate, A (Fig. 4) describe a circle in 



