68 HISTORY OF OPTICS. 



scopes, both with reference to spherical and to chromatic abei 

 rations. 



According to Dollond's discovery, the colored spectra produced b} 

 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 colors, yellow, green, <kc., in corresponding places in 

 the two? This point also could not be determined any otherwise 

 than by experiment. It appeared that such a correspondence did not 

 exist ; and, therefore, when the extreme colors were corrected by 

 combinations of the different media, there still remained an uncor- 

 rected residue of color arising from the rest of the spectrum. This 

 defect was a consequence of the property, that the spectra belonging 

 to different media were not divided in the same ratio by the same 

 colors, and was hence termed the irrationality of the spectrum. By 

 using three prisms, or three lenses, three colors may be made to coin- 

 cide instead of two, and the effects of this irrationality greatly 

 diminished. 



For the reasons already mentioned, we do not pursue this subject 

 further, 5 but turn to those optical facts which finally led to a great 

 and comprehensive theory. 



[2nd Ed.] [Mr. Chester More Hall, of More Hall, in Essex, is 

 said to have been led by the study of the human eye, which he con- 

 ceived to be achromatic, to construct achromatic telescopes as early 

 as 1729. Mr. Hall, however, kept his invention a secret. David 

 Gregory, in his Castries (1713), had suggested that it would perhaps 

 be an improvement of telescopes, if, in imitation of the human eye, 

 the object-glass were composed of different media. Encyc. Brit. art. 

 Optics. 



It is said that Clairaut first discovered the irrationality of the 

 colored spaces in the spectrum. In consequence of this irrationality, 

 it follows that when two refracting media are so combined as to cor- 

 rect each other's extreme dispersion, (the separation of the red and 

 violet rays,) this first step of correction still leaves a residue of colora- 



s The discovery of the fixed lines in the spectrum, by Wollaston and Fraun- 

 hofer, has more recently supplied the means of determining, with extreme 

 accuracy, the corresponding portions of the spectrum in different refracting 

 substances. 



