1G 



A POPULAR ACCOUNT 



prism. In this case the eye supplied the 

 place of the screen in the former experi- 

 ment, the rays which formed the spec- 

 trum entering the pupil as they emerged 

 from the prism. He found the same 

 effects to ensue. He beheld the oblong 

 spectrum as before, the order of the 

 colours being in all respects the same. 



(24.) It appears by the experiment 

 which we have described that the action 

 of the prism was such as to dilate the 

 ray in a direction at right angles to its 

 length, and thereby to give the spectrum 

 the oblong form. Now if this dilatation 

 was the consequence of the action of the 

 material of the prism, and not of the 

 various refrangibility of the component 

 parts of solar light, it must inevitably 

 follow that a second prism placed with 

 its length vertical, and consequently at 

 right angles to the first, so as to refract 

 the light sideways, would dilate the ray 

 as much in the horizontal direction as 

 the first did in the vertical. The infer- 

 ence from this would necessarily be, that 

 the combined action of two prisms would 

 be to give a square spectrum, as much 

 length being obtained by the action of 

 the one, as breadth by the action of the 

 other. 



Accordingly Newton tried this expe- 

 riment. Let RV,y2g-. 15, be the spec- 

 trum produced by the 

 first prism. He placed 

 another prism with its 

 length vertical, and 

 consequently at right 

 angles to the first, and 

 when the rays were in- 

 tercepted by it, the ef- 

 fect was, that instead 

 of the spectrum, V R, 

 being spread over a square surface, it 

 retained its breadth, but was transferred 

 to the position V'R/. 



This result furnished a most convinc- 

 ing and beautiful confirmation of the 

 theory of Newton. The rays at the 



j 5 

 " 



upper end of the spectrum were deflected 

 by the second prism, through the dis- 

 tances A B, from their former position, 

 while the rays at the lower end were 

 only deflected through the smaller spaces 

 a b. The breadth of the spectrum re- 

 mained unaltered, plainly shewing that 

 the second prism had no power to dilate 

 the rays which formed it. The rays 

 which were most refracted by the first 

 prism, were those of the blueish charac- 

 ter, which occupied the upper part of 

 the spectrum. These same rays were 

 also most refracted by the second prism, 

 being most removed from their first po- 

 sition (through the spaces A B). Also 

 the rays which were least refracted by 

 the first prism, were those of the reddish 

 character which occupied the lower part 

 of the spectrum ; and these also are least 

 refracted by the second prism, being 

 those which are least removed from their 

 places through the distances a b. 



To put this question even more beyond 

 dispute, Newton received the rays from 

 the second prism on a third, placed with 

 its length parallel to the length of the 

 spectrum, and found the same effect re- 

 peated, the third position of the spec- 

 trum being inclined to the second in the 

 same manner as the second was inclined 

 to the first, but no dilatation taking 

 place, and the breadth of the spectrum 

 remaining the same. He states that he 

 used a fourth prism with the same re- 

 sult. 



It is important to remember that in all 

 these experiments the light is all inci- 

 dent on each prism at the same angle. 

 For if its parts fell upon the surface at 

 different angles, different quantities of 

 refraction would be the natural and ne- 

 cessary result. 



(25.) Newton contrived a very elegant 

 experiment to shew the regularity with 

 which the prisms determined the magni- 

 tude, figure, and position of the spectrum. 

 Before two small apertures F,/, fig. 16, 



in the window shutter he placed two 

 similar prisms A B C, a b c, the one im- 



mediately beneath the other, with their 

 lengths parallel and horizontal. These 



