PHYSICS OF THE NINETEENTH CENT URY.-LIGffT. 447 



than the aqueous humour, and it increases in density towards its 

 centre. The larger cavity of the eyeball c is completely filled with a 

 watery fluid called the vitreous humour, and its inner surface on the 

 back part is lined by the retina a delicate network of nerves supported 

 by a dark-coloured layer called the choroid. The pencils of rays which 

 enter the pupil of the eye are rendered more convergent by the refrac- 

 tion of the crystalline lens, and are collected into foci on the retina, 

 where they form an inverted image according to the law of refraction 

 by lenses (page 157). 



Young applied the mathematical formulae of optics to calculating 

 the refractions of the various humours of the eye, and he determined 

 by ingenious methods the necessary experimental data. He noticed 

 that a small luminous spot, such as a star or the image of a candle in a 

 small concave mirror, never appears perfect in form, but always pre- 

 sents itself with the radiated appearance. These radiating lines he 

 attributed to inequalities in the re- 

 fraction, occasioned by the fibrous 

 structure of the crystalline lens. 

 Fig. 200 displays the arrangement 

 of the superficial layer of fibres in 

 the crystalline lens of the human 

 eye, and the six-rayed figure, divided 

 into branches, corresponds with the 

 appearances presented by luminous 

 points. Young mentions an ele- 

 gant and very simple experiment 

 by which Dr. Wollaston proved that 

 the eye is not truly achromatic. He 

 looks at a luminous point (e.g. t a 

 brgiht star) through a prism, which 

 then gives, of course, a spectrum 

 (page 217) having no apparent breadth. Nevertheless, the eye not 

 being able to bring at one time the several rays of this linear spectrum 

 to a focus, it happens that when the red extremity is seen distinctly as 

 a point, the blue rays will, by the same adjustment of the eye, be too 

 much refracted, and will appear expanded into a surface ; when, on 

 the other hand, the focus is adapted for the blue extremity, the red end 

 appears expanded : in either case the line will appear as a narrow 

 triangle. That the accommodation of the eye to view objects at dif- 

 ferent distances is effected by a change in the curvature of the crystal- 

 line lens Young had now satisfied himself by his own observations and 

 those of others. 



In the Bakerian Lecture for 1801 Young drew attention to the theory 

 of undulations as explaining the phenomena of light. He did not so 

 much propound a new theory as support by additional evidence one 

 which had been already proposed ; and he applied it to the explanation 



FIG. 200. 



