CONTRIBUTIONS TO SCIENCE. 483 



with that gained by the other, and thus to state in the 

 language of those possessing normal vision, how colours 

 really appear to the colour-blind. His results agree in 

 their general character with Maxwell's, but Holmgren 

 (with many others) makes the third sensation correspond 

 to violet instead of blue, while, according to him, the 

 violet -blind class colours as red and yeitow" instead of 

 red and green. 



Though the largest and most important, Maxwell's theory 

 of compound colours was by no means his only, or even his 

 earliest, contribution to Optics. While a bachelor-scholar at 

 Trinity, at the request of Messrs. Macmillan he wrote a con- 

 siderable portion of a text-book on geometrical optics. The 

 work was never finished, but the MS. is still extant. The 

 mode of treating the subject, as stated by Professor Maxwell 

 in one of his letters to his father, 1 is decidedly novel, and 

 calculated to bring down a storm of abuse upon the author. 

 He starts by postulating the possibility of obtaining perfect 

 images, and then investigates the laws of reflection and 

 refraction necessary for this. 



Perhaps the most remarkable of Maxwell's contributions 

 to Optics was his identification of the velocity of light 

 with the ratio of two quantities, each of which is capable 

 of being measured electrically; but this subject will be 

 again referred to in describing his electrical researches. 

 A paper on a general theory of optical instruments was 

 published by Maxwell in the Quarterly Journal of Mathe- 

 matics in 1858, and another on the same subject in the 

 Proceedings of the Cambridge Philosophical Society, 1866. 

 A paper on the best mode of projecting a spectrum on a 

 screen appeared in the Proceedings of the Royal Society of 

 Edinburgh in 1869. We may also refer to the papers on 

 " The Focal Lines of a refracted Pencil," in the Proceedings of 

 the Mathematical Society of London for 1871-3' on a" Bow 

 seen on the surface of ice," in the Proceedings of the Royal 

 Society of Edinburgh for 1872; on " The Spectra of Polarised 



1 See Part I. p. 217. 



