1 66 On the Decomposition of Light, 



debted for a very interesting collection of experiments upon 

 this subject. 



In some of these he determines, with a great deal of care, 

 the dispersive power of various substances. He finds some 

 of them which possess it in a degree considerably stronger 

 than that of common or crown glass ; such as, for example, 

 sal-ammoniac, the muriatic acid in particular, and, above 

 all, the smoking muriate of antimony. But what is not 

 less remarkable, is, the displacing of the limits of the co- 

 Jours in the dispersions caused by these different substances; 

 for the less dispersive substances, in gtaeral, cause the mean 

 direction to pass by the junction of the green and the blue; 

 while other substances, tuch as Jlint glass, essential oils, 

 and metallic solutions, throw the limit of these same colours 

 much nearer the red ; and it is, on the contrarv, carried 

 again towards the violet upon the spectrum produced by 

 the muriatic acid. These varieties induced Dr. Blair to 

 prepare object glasses more truly achromatic than the com- 

 mon ones. He made them of several forms, one, among 

 the rest, by means of an assemblage of curved glasses, be- 

 tween which he enclosed a solution of muriate of autiniony, 

 mixed with muriatic acid, to the precise quantity necessary 

 for compensating by its action, for the displacing of the 

 green rays, which the other substance would have occa- 

 sioned. 



In short, we i"nay form an idea of the (comparative ex- 

 actitude of the methods of Dollond and Blair, by means of 

 two very simple figures, on which we trace the course of 

 the rays, with reference to the axis of each object glass. 

 There will result, 1st, That object glasses made bv these 

 two processes are both susceptible of giving a circular focus 

 of white light exempt frona iris, but a little broader in Dol- 

 lond's : 2d, Dr. Blair's does not necessarily unite all the 

 kinds of rays, as he thinks it does : this does not lessen, 

 however, the practical advantages of the instrument, for the 

 perfection of which wc arc indebted to him. 



I have dwelt some time upon these details, because they 

 appeared to me to be connected with n>y subject : f shall 



now 



