94 Mr. Cooi^er on the Colours that enter into the 



numerous data furnished by the author of the new biography. 

 It remains now for us to consider him as a mathematician, 

 and thank God our task will be henceforth much more 

 easy ; for, the method of re-calling him to memory in this 

 point of view, the only one which, in our opinion, merits 

 the attention of posterity, will be to make extracts from his 

 own works, completed by several new and curious docu- 

 ments which his new biographer has added. In this respect 

 we may say, with justice, that this biography has been 

 highly usefuL 



(To be continued.) 



Article II. 

 On the Number and Character of the Colours that enter into 

 the Composition of White Light. By Paul Cooper, Esq. 

 ( Concluded from p. 64. ) 



It is a well known fact that, with common light, refraction 

 is always attended by partial reflexion ; and it is equally 

 well known that the internal reflexion from glass, is stronger 

 than the external ; even under ordinary circumstances this 

 reflexion is considerable, and with such intense light as the 

 direct light of the sun, at a large angle of incidence, such 

 as it is in the present experiment, it must be sufficient to 

 produce very decided effects. The question, then, is, Do 

 these reflexions, traced upon optical principles, correspond 

 with the intermediate images discovered in the spectrum ? 



This is the question which I shall now endeavour to 

 answer : Let ABC {Fig. 3)* represent the section of an 

 equilateral prism, perpendicular to the axis, and E F G H 

 a ray of homogeneous light, incident upon the centre of the 

 surface A C, at such an angle that F G may be parallel to 

 A B, and, consequently, B G H equal to A F E ; in this case 

 it is evident that the first internal reflexion will be from 

 G to D, the second from D to F, and the third from F to G, 

 where it will fall at the same angle of incidence as the 

 direct ray F G, and, of course, be equally refracted upon 

 emergence in the direction G H. The fourth reflexion will 

 take the same direction as the first ; and, proceeding in the 

 same order, the sixth will coincide with the third, forming 

 upon emergence at G, with the direct ray, and rays from 



* See pao-p .59. 



