DISCOVERY OF ACHROMATISM. 397 



mathematically the conditions requisite for such a 

 result. Klingenstierna 3 , a Swedish mathematician, 

 also showed that Newton's rule could not be uni- 

 versally true. Finally, John Dollond 4 , in 1757, re- 

 peated Newton's experiment, and obtained an oppo- 

 site result. He found that when an object was seen 

 through two prisms, one of glass and one of water, 

 of such angles that it did not appear displaced by 

 refraction, it was coloured. Hence it followed that, 

 without being coloured, the rays might be made to 

 undergo refraction ; and that thus, substituting 

 lenses for prisms, a combination might be formed, 

 which should produce an image without colouring 

 it, and make the construction of an achromatic 

 telescope possible (HA). 



Euler at first hesitated to confide in Dollond's 

 experiments ; but he was assured of their correct- 

 ness by Clairaut, who had throughout paid great 

 attention to the subject; and those two great ma- 

 thematicians, as well as D'Alembert, proceeded to 

 investigate mathematical formula which might be 

 useful in the application of the discovery. The 

 remainder of the deductions, which were founded 

 upon the laws of dispersion of various refractive 

 substances, belongs rather to the history of art than 

 of science. Dollond used at first, for his achromatic 

 object-glass, a lens of crown-glass, and one of flint- 

 glass. He afterwards employed two lenses of the 

 former substance, including between them one of 



3 Swedish Trans. 17">1. 4 Phil. Trans. 



