Her. m] DISSERTATION SECOND. 85 



vex, for instance, and another concave, there was a ten- 

 dency of the two contrary dispersions to correct one 

 another. But it appeared to Newton, on examining dif- 

 ercnt refracting substances, that the dispersion of the 

 coloured rays never could be corrected except when the 

 refraction itself was entirely destroyed, for he thought 

 he had discovered that the quantity of the refraction and 

 of the dispersion in different substances bore always the 

 same proportion to one another. This is one of the few 

 instances in which his conclusions have not been confirm- 

 ed by subsequent experiment ; and it will, accordingly, fall 

 under discussion in another part of this discourse. 



Having taken the resolution of constructing a reflecting 

 telescope, he set about doing so with his own hands. 

 There was, indeed, at that time, no other means by 

 which such a work could be accomplished ; the art of the 

 ordinary glass-grinder not being sufficient to give to me- 

 tallic specula the polish which was required. It was on 

 this account that Gregory had entirely failed in realizing 

 his very ingenious optical invention. 



Newton, however, himself possessed excellent hands for 

 mechanical operations, and could use them to better pur- 

 pose than is common with men so much immersed in 

 deep and abstract speculation. It appears, indeed, that 

 mechanical invention was one of the powers of his mind 

 which began to unfold itself at a very early period. In 

 some letters subjoined to a memoir drawn up after his 

 death by his nephew Conduit, it is said, that, when a boy, 

 Newton used to amuse himself with constructing machines, 

 mills, &c. on a small scale, in which he displayed great 

 ingenuity ; and it is probable that he then acquired that 

 use of his hands which is so difficult to be learned at a 

 later period. To this, probably, we owe the neatness 

 and insenuitv with which the optical experiments abovr 



