DISCOVERY OF ACHROMATISM. 399 



was a consequence of the property, that the spectra 

 belonging to different media were not divided in 

 the same ratio by the same colours, and was hence 

 termed the irrationality of the spectrum. By using 

 three prisms, or three lenses, three colours may be 

 made to coincide instead of two, and the effects of 

 this irrationality greatly diminished (IA). 



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 com- 

 prehensive theory. 



5 The discovery of i\\ejixed lines in the spectrum, by Wollas- 

 ton and Fraunhofer, has more recently supplied the means of 

 determining, with extreme accuracy, the corresponding portions 

 of the spectrum in different refracting substances. 



