12 



A POPULAR ACCOUNT 



rays R and B, with the perpendiculars 



P to the refracting surface will be equal, 



Fig. 5. 



because the rays R and B are parallel. 

 On entering the medium they will be 

 both deflected by the law of refraction 

 towards the perpendiculars p. Let r be 

 the course after refraction of the red 

 ray, ? and b the course after refraction of 

 the blue one. If these rays were equally 

 refrangible they would be equally de- 

 flected towards the perpendicular. Such, 

 however, is not the case. The red ray 

 r will be less deflected from its original 

 direction than the blue ray b, and there- 

 fore the angle under the lines r and p 

 will be greater than the angle under b 

 and p. Hence, red light is said to be 

 less refrangible than blue light. 



We might expect that this effect 

 would be easily reduced to experiment 

 by colouring two sticks, one red, and 

 the other blue, and placing them paral- 

 lel to each other, obliquely in water. In 

 this case, the broken appearance which 

 the sticks should exhibit would be dif- 

 ferent, the angle formed by the parts in 

 the water with those in the air being 

 more obtuse in the red than in the blue 

 stick. It happens, however, that the 

 difference between their deflections is 

 so small, that, in this way, it would be 

 very difficult to render it perceptible. 



To obviate this inconvenience, and 

 render the difference of refractions so 

 great as to be easily perceptible, Newton 

 contrived that the different lights should 

 each be twice refracted, so that being 

 originally parallel, they should thus ac- 

 quire the sum of the divergencies which 

 such refraction alone would give them. 

 This he accomplished by using a glass 

 prism, in the manner which we shall 

 now describe.* 



* We advise every reader who has not seen the 

 prismatic spectrum, before he proceeds further, to 

 procure a prism, or a common angular piece of 

 glass with plane sides, and to transmit through it a 

 beam of the sun's light admitted through an aperture 

 in . e window.shntter of a dark room. The facility 



th which he will comprehend Newton's interesting 

 experiments will thus be much increased. 



( 1 7. ) Let ABC, Jig. 6, represent a sec- 

 tion of a triangular glass prism at right 



Fig. 6. 



B 



angles to its axis (an end view of it.) A 

 ray of red light entering at I in the direc- 

 tion RI will, by the refraction of the 

 glass, be deflected in the direction r, to- 

 wards the perpendicular to the surface 

 B C, on which it is incident. On meet- 

 ing the second surface B A, it will emerge, 

 suffering a second refraction on passing 

 into the air. But here, as it passes from 

 glass into air, it will be deflected from 

 the perpendicular to the surface B A in 

 the direction /, which increases still 

 more its deviation from its original course 

 R. In like manner, a blue ray incident 

 in the same direction at I will suffer two 

 deflections at the surfaces of the prism ; 

 but in each case will be more deflected 

 from its original direction R, than the 

 red ray. The deviation of the blue from 

 the red ray, on emerging from the sur- 

 face B A, will evidently be found, by 

 adding together the two deviations at 

 the surfaces B C and B A. Thus, al- 

 though either deviation alone might be 

 too small to be perceptible, yet the sum 

 of both will produce a sensible effect. 



( 1 8.) To reduce this to absolute experi- 

 ment, Newton states that he took a black 

 oblong stiff paper, terminated by parallel 

 sides, with a line drawn from side to 

 side, dividing it into two equal parts. 

 One of these was coloured with an in- 

 tense red, and the other with an intense 

 blue. He then covered the wall and 

 shutters, surrounding the window of the 

 room with black cloth, to prevent the 

 light reflected from them from interfer- 

 ing with his experiment. On a table 

 before the window, also covered with 

 black, he placed the coloured paper, the 

 line dividing the colours being perpendi- 

 cular to the plane of the window. Things 

 being thus arranged, he held the prism, 

 with its angle B,y?g-. 7, upwards, so that 

 the light coming from the paper RB 

 should be twice refracted before it reach- 

 ed the eye placed behind the prism, as 

 represented in the cut. Upon viewing 

 the appearance through the prism, he 

 found that the blue half of the paper 

 appeared a little more elevated than the 



