DI3COVEKY OF ACHROMATISM. 07 



is white after refraction, when the emergent rays are parallel to the 

 incident, and in no other case. If this were so, the production of 

 colorless images by refracting media would be impossible ; and such, 

 .'n deference to Newton's great authority, was for some time the 

 general persuasion. Euler observed, that a combination of lenses 

 which does not color the image must be possible, since we have an 

 example of such a combination in the human eye ; and he investigated 

 mathematically the conditions requisite for such a result. Klingen- 

 stierna, 3 a Swedish mathematician, also showed that Newton's rule 

 could not be universally true. Finally, John Dolloncl, 4 in I'ZoT, 

 repeated Newton's experiment, and obtained an opposite 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 colored. Hence it followed that, without being- 

 colored, 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 coloring it, and make the 

 construction of an achromatic telescope possible. 



Euler at first hesitated to confide in Dollond's experiments ; but he 

 was assured of their correctness by Clairaut, who had throughout paid 

 great attention to the subject ; and those two great mathematicians, 

 as well as D'Aiembert, proceeded to investigate mathematical for- 

 mulas 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 his- 

 tory 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 after- 

 wards employed two lenses of the former substance, including between 

 them one of the latter, adjusting the curvatures of his lenses in such a 

 way as to correct the imperfections arising from the spherical form of 

 the glasses, as well as the fault of color. Afterwards, Blair used fluid 

 media along with glass lenses, in order to produce improved object- 

 glasses. This has more recently been done in another form by Mr. 

 Barlow. The inductive laws of refraction being established, their 

 results have been deduced by various mathematicians, as Sir J. Hcr- 

 schel and Professor Airy among ourselves, who have simplified and 

 extended the investigation of the formulae which determine the best 

 combination of lenses in the object-glasses and eye-glasses of tele- 



2 Ac. Berlin. 1747. 3 Swedish Trans. 175-1. 4 Phil. Trans. 1758 



