162 Sir David Brewster*s Ohseivations on 



the same focus as parallel red rays. In support of this opinion 

 I may adduce the experimental testimony of Dr. WoUaston 

 and Dr. Young. In order to prove " the dispersive power of 

 the eye," Dr. WoUaston " looks through a prism at a small 

 lucid point, which of course becomes a linear spectrum. But 

 the eye cannot so adapt itself as to make the whole spectrum 

 appear a line; for if the focus be adapted to collect the red 

 rays to a point, the blue will be too much refracted, and ex- 

 pand into a surface ; and the reverse will happen if the eye be 

 adapted to the blue rays; so that in either case the line will be 

 seen as a triangular space." To this interesting observation 

 Dr. Young adds the following experiments. " The observa- 

 tion is confirmed by placing a small concave speculum in dif- 

 ferent parts of a prismatic spectrum, and ascertaining the 

 utmost distances at which the eye can collect the rays of 

 different colours to a focus. By these means I find that the 

 red rays, from a point at 1 2 inches^ distance, are as much re- 

 fracted as white or yellow light at II. The difference is equal 

 to the refraction of a lens 1 32 inches in focus*. " 



In a subsequent paper, " On some cases of the Production of 

 Colour^^ (Lectures, vol. ii. p. G38,) Dr. Young informs us that 

 he has confirmed his previous observations on the dispersive 

 powers of the eye : " I find," says he, " that at the respective 

 distances of 10 and 15 inches the extreme red and the extreme 

 violet rays are similarly refracted, the difference being ex- 

 pressed by a focal length of 30 inches. Now the interval be- 

 tween red and yellow is about one fourth of [the whole spec- 

 trum ; consequently, a focal length of 120 inches expresses a 

 power equivalent to the dispersion of the red and yellow, and 

 this differs but little from 132, which was the result of the ob- 

 servation already described. I do not know that these ex- 

 periments are more accurate than the former one; but I have 

 repeated them several times under different circumstances, 

 and I have no doubt that the dispersion of coloured light in 

 the human eye is nearly such as I have stated it. It may also 

 be ascertained very accurately, by looking through an aper- 

 ture, of known dimensions, at the image of a point dilated by 

 a prism into a spectrum, and measuring the angle formed by 

 its sides on account of the difference of refrangibility of the 

 rays ; and this method seems to indicate a greater dispersive 

 power than the former." 



When Dr. Wells, as quoted by Mr. Powell, states " that 

 the eye has no principle of achromatic compensation in its lens, 

 since the refractions are all performed one way," he would be 



* On the Mechanism of the Eye: Lectures, vol. ii. pp. 584, 585. 



