270 PHILOSOPHICAL TRANSACTIONS. [aNNO 1758. 



had the satisfaction to find, as he had expected, free from the errors arising from 

 the different refrangibility of light : for the refractions by which the rays were 

 brought to a focus, were every where the differences between two contrary re- 

 fractions, in the same manner, and in the same proportions as in the experi- 

 ment with the wedges. 



However, the images formed at the foci of these object-glasses, were still very 

 far from being so distinct as might have been expected from the removal of so 

 great a disturbance ; and yet it was not very difficult to guess at the reason, 

 when it is considered, that the radii of the spherical surfaces of those glasses 

 were required to be so short, in order to make the refractions in the required 

 proportions, that they must produce aberrations or errors in the image, as great, 

 or greater than those from the different refrangibility of light. And therefore, 

 seeing no method of getting over that difficulty, he gave up all hopes of succeed- 

 ing in that way. And yet, as these experiments clearly proved, that different 

 substances diverged the light very differently in proportion to the refraction ; he 

 began to suspect that such variety might possibly be found in different sorts of 

 glass, especially as experience had already showed that some made much better 

 object-glasses in the usual way than others : and as no satisfactory cause had as 

 yet been assigned for such difference, there was great reason to presume, that it 

 might be owing to the different divergency of the light by their refractions. 



Therefore the next business to be undertaken, was to grind wedges of different 

 kinds of glass, and apply them together, so that the refractions might be made 

 in contrary directions, in order to discover, as in the foregoing experiments, 

 whether the refraction and divergency of the colours would vanish together. 

 But a considerable time elapsed before he could set about that work ; for though 

 he was determined to try it at his leisure, for satisfying his own curiosity, yet he 

 did not expect to meet with a difference sufficient to give room for any great im- 

 provement of telescopes ; so that it was not till the latter end of the year that he 

 undertook it, when his first trials convinced him that this business really deserved 

 his utmost attention and application. 



He discovered a difference far beyond his hopes, in the refi-active qualities of 

 different kinds of glass, with respect to their divergency of colours. The yellow 

 or straw-coloured foreign sorts, commonly called Venice glass, and the English 

 crown glass, are very near alike in that respect, though in general the crown 

 glass seems to diverge the light rather the less of the two. The common plate 

 glass made in England diverges more; and the white crystal or flint English 

 glass, as it is called, most of all. It was not now his business to examine into 

 the particular qualities of every kind of glass that he could come at, much less 

 to amuse himself with conjectures about the cause, but to fix on such two sorts 

 as their difference was the greatest; which he soon found to be the crown, and 



