444 HISTORY OF SCIENCE. 



was a subject of some difficulty, namely, the means by which the eye 

 is adapted for perceiving objects whether distant or near, so that the 

 image is brought to a proper focus on the retina. Some philosophers 

 had supposed that the ball of the eye, as a whole, was capable of 

 changing its form, so as to become more or less flattened as occasion 

 required ; others had contended that the crystalline lens was capable 

 of the same kind of movement as the lens of the camera obscura , 

 others, that the curvature of the external part of the eye, called the 

 cornea, changed, in order to give the necessary optical adjustment; 

 others, again, that a change took place in the curvature of the crystal- 

 line lens. This last explanation had been the subject of discussion 

 before Young advocated it ; but Sir Everard Home, the eminent ana- 

 tomist, and Ramsden, the celebrated instrument maker, conjointly 

 decided against this view, and declared that the crystalline lens was 

 unalterable ; and Young himself, deferring to authorities so great, 

 thought that he must have been mistaken. But a few years afterwards 

 he withdrew this retractation, and in a paper published in 1800 he 

 enters very fully into numerous observations and direct experiments in 

 support of his views. 



Young tells us that in 1801, when reflecting on the beautiful optical 

 experiments of Sir Isaac Newton, he discovered a law which appeared 

 to him to account for a greater variety of interesting phenomena than 

 any principle that had then been made known. What this principle 

 was he explains by the following comparison : " Suppose a number of 

 equal waves of water to move upon the surface of a stagnant lake with 

 a certain constant velocity, and to enter a narrow channel leading out 

 of the lake; suppose, then, another similar cause to have excited 

 another equal series of waves, which arrive at the same channel with 

 the same velocity and at the same time with the first. One series of 

 waves will not destroy the other, but their effects will be combined. 

 If they enter the channel in such a manner that the elevations of the 

 one series coincide with those of the other, they must together produce 

 a series of greater joint elevations; but if the elevations of one series 

 are so situated as to correspond to the depressions of the other, they 

 must exactly fill up those depressions, and the surface of the water 

 must remain smooth, at least, I can discover no alternative, either 

 from theory or from experiment. Now, I maintain that similar effects 

 take place whenever two portions of light are thus mixed, and this I 

 call the general law of the interference of light" It is in these terms 

 that Young first unfolded his beautiful discovery of the interference of 

 light, concerning which we shall have more to say presently. 



About 1 80 1 Young became connected, as Professor of Natural 

 Philosophy, with the Royal Institution, which had then been recently 

 established, and has since been made famous by the distinguished men 

 who have filled its chairs by Young and Davy, Faraday and Tvji^all. 

 No man of the present century has displayed a wider range of ac- 



