398 HISTORY OF OPTICS. 



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 colour. 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 mathema- 

 ticians, as Sir J. Herschel and Professor Airy among 

 ourselves, who have simplified and extended the 

 investigation of the formula which determine the 

 best combinations of lenses in the object-glasses 

 and eye-glasses of telescopes, both with reference to 

 spherical and to chromatic aberrations. 



According to Dollond's discovery, the spectra 

 produced by prisms of two substances, as flint-glass 

 and crown-glass, would be of the same length when 

 the refraction was different. But a question then 

 occurred : When the whole distance from the red 

 to the violet in one spectrum was the same as the 

 whole distance in the other, were the intermediate 

 colours, yellow, green, &c. in corresponding places 

 in the two? This point also could not be deter- 

 mined any otherwise than by experiment. It ap- 

 peared that such a correspondence did not exist; 

 and, therefore, when the extreme colours were cor- 

 rected by combinations of the different media, there 

 still remained an uncorrected residue of colour 

 arising from the rest of the spectrum. This defect 



