84 DISSERTATION SECOND. [part a. 



MM. Linus and Lucas, both of Leige, objected to 

 Newton's experiments as inaccurate ; the tirst, because, 

 on attempting to repeat them, he had not obtained the 

 game results ; and the second, because he had not been 

 able to perceive that a red object and a blue required 

 the focal distance to be different when they were viewed 

 through a telescope. Newton replied with great patience 

 and good temper to both. 



The series was closed, in 1727, by the work of an 

 Italian author, Rizetti, who, in like manner, called in 

 question the accuracy of experiments which he himself 

 had not been able to repeat. Newton was now no more, 

 but Desaguliers, in consequence of Rizetti's doubts, insti- 

 tuted a series of experiments which seemed to set the 

 matter entirely at rest. These experiments are described 

 in the Philosophical Transactions for 1728. 



An inference which Newton had immediately drawn 

 from the discoveries above described was, that the great 

 source of imperfection in the refracting telescope was the 

 different refrangibility of the rays of light, and that there 

 were stronger reasons than either Mersenne or Gregory 

 had suspected, for looking to reflection for the improve- 

 ment of optical instruments. It was evident, from the 

 different refrangibility of light, that the rays coming from 

 the same point of an object, when decomposed by the 

 refraction of a lens, must converge to different foci ; the 

 red rays, for example, to a point more distant from the 

 lens, and the violet to one nearer by about a fifty-fourth 

 part of the focal distance. Hence it was not merely 

 from the aberration of the rays caused by the spherical 

 figure of the lens, that the imperfeetion of the images 

 formed by refraction arose, but from the very nature of 

 refraction itself. It was evident, at the same time, that 

 in a combination of lenses with opposite figures, one con- 



