October 25, 1895.] 



SCIENCE. 



561 



bles and other thiu trausparent substances ; also 

 he explained phenomena not previously known, 

 such as the shadow produced on a screen by the 

 superposition of two luminous rays proceeding 

 from the same source and arriving at the same 

 point by different routes of unequal length ; or, 

 again, the augmentation of light which appears 

 periodically when the differences of the respec- 

 tive routes varj\ He saw that these facts were 

 admirably explained by the hypothesis of un- 

 dulations in an imponderable medium which, 

 according to the concordance or discordance of 

 their periods, mutually added to or destroyed 

 each other ; an hypothesis, it will be seen, 

 diametrically opposed to that advanced by 

 Newton. Notwithstanding his endeavor to 

 place his idea under the patronage of his illus- 

 trious predecessor, the theory of interferences 

 was denounced as sacriligious by powerful 

 adversaries, who held the public conscience 

 and who never relinquished their hostile atti- 

 tude. 



But Young was ' not without honor save in his 

 own country ; ' the recognition denied him in 

 England has, thanks to a happy accident, been 

 freely accorded in Prance. It was at the hands 

 of the great Arago, himself interested in 

 optics, that he received his first encouragement, 

 followed by academic honors. In any case, it 

 is in the works of Fresuel, who was inspired by 

 the same principle, that Young's achievements 

 were for the first time crowned with approbation; 

 and it was the experiments Of Foucault on the 

 rapidity of the propagation of light through air 

 and through water which gave the fatal blow 

 to the emission theory and secured for the 

 theory of interference a final victory. For 

 some time past the physicists have shown a cer- 

 tain indifference concerning the undulatory and 

 molecular theories, doubtless because of the 

 superficial generalization built upon these theo- 

 ries twenty years ago. We cannot refuse to 

 them, however, in spite of their hypothetical 

 character, a marvellous role in discovery (as, for 

 instance, the experiments of Herz on electrical 

 undulation) as well as a system of representa- 

 tion quite satisfactory to the mind which does 

 not seek to make a universal application. How- 

 ever this may be, Young has other claims to 

 the admiration of physicists, as, for instance, 



the first efforts made to unite by an empirical 

 formula the elastic forces of vapors and their 

 temperatures. 



Tlie works of Young in ophthalmology have 

 not attracted the attention they merit, notwith- 

 standing the liberal recognition of their results 

 on the part of Helmholtz and Bonders. Max- 

 well, Helmholtz and Rood have popularized his 

 theory of colors which consists in attributing 

 to the retina three kinds of fibres responding 

 respectively to the three fundamental colors : 

 red, green and violet ; and producing by 

 their combined impression the sensation of all 

 the other colors ; in like manner the combina- 

 tion of these three primary colors in variable 

 proportions is able to produce all the appear- 

 ance of colors. This hypothesis admits but a 

 single mode of transmission of the impression 

 through the nerves to the brain. It makes the 

 differentiation of the sensation of color consist 

 in a functional modification of the periphery 

 rather than in a cerebral operation. But, while 

 it offers a plausible explanation of certain cases 

 of color-blindness, this theory is far from having 

 obtained general acceptance. It seems difficult 

 to apply it to the explanation of those cases of 

 color-blindness where but a single color is per- 

 ceived and that different from the fundamental 

 colors of Young. But it must be admitted that 

 Young did not advance his hypothesis without 

 reserve, and the theories which have since un- 

 dertaken to supersede his are scarcely more 

 satisfactory; they are not susceptible of any 

 better experimental proof nor of theoretical de- 

 ductions. 



A distinguished ophthalmologist, M. Tschern- 

 ing, has just published a French translation of 

 Young's works on ophthalmology with liberal 

 commentary. The subjects therein treated ap- 

 pear in the following order : 



1. Observations on Vision. (Philos. Trans, 

 for 1793.) 



2. A memoir ' On the Mechanism of the Eye.' 

 (Philos. Trans, for 1801.) 



3. Extracts on the vision of colors taken 

 from different essays. 



4. The 38th chapter of Lessons on Natural 

 Philosophy, London, 1807 : On Vision. 



' Observations on Vision ' were written at the 

 age of twenty and present many remarkable re- 



