56 Mr. S. P. Tliompson on the Chromatic Aberration 



violet rays freely, but totally opaque to all rays between D 

 and E, and nearly opaque to rays from C to D in the orange, 

 and from E to F in the green and blue. 



27. A silvered bead placed in the sunlight, and viewed 

 through such a medium can by no possibility be seen with 

 accurate focus for all rays. It appears either as a near red 

 point of light surrounded by a blue haze, or as a distant 

 blue light surrounded by a red haze *. The point of light 

 seems to change distance from far to near, or from near to far, 

 with the effort of the eye. This effect, however, is only to 

 be well observed when the intensities of the red and blue rays 

 are about equal ; for if either predominate, the eye will focus 

 for the brighter unless special care is taken to adjust and keep 

 the required focus. 



28. The researches of Jurin, Schreiner, Miiller, Powell, and 

 Trouessart have tended in favour of the view that the eye is 

 achromatic at least for objects at the centre of the field of 

 vision, when distinctly in focus. The fundamental illustration 

 of this view is as follows : — Take any visible white object — say 

 a square inch of white paper upon a black background. If 

 the eye be focused for a distance beyond the white surface, 

 it will appear blurred at the edges, being tinged outwardly 

 with orange-yellow, inwardly with blue. But if the eye be 

 focused for a point nearer than the white surface, it will ap- 

 pear tinged outwardly with blue, inwardly with orange-yellow, 

 while at exact focus these bordering tints disappear. To 

 explain this seeming achromatism of the eye it was argued 

 by Wollaston, Young, Miiller, and Matthiesen that the rays 

 passed almost without refraction along the axis of the eye, and 

 therefore suffered no dispersion f- On the other hand, in my 

 own experiments with permanganate-of-potash solution to 

 intercept the yellow and green rays, I have never been able 

 to obtain a luminous surface small enough to be free from aber- 

 ration at the edges — which proves that the focus is really not 

 exact. The rays of the middle of the spectrum, being more 

 intense, mask the effects of the feebler rays ; and the eye 

 focuses for the more intense light, as I have mentioned in 

 § 27. 



29. If, on the contrary, a weak solution of picrate of 

 potash be employed as a screen to cut off the extreme red 



* This effect may be shown objectively to an entire audience hy casting 

 beams of light from a lamp through such a purple medium upon a silvered 

 ball. 



t Baden Powell contended that the refraction and dispersion of the 

 vitreous humour being in the opposite sense to that of the crystalline lens, 

 aqueous humour, and cornea, compensated the dispersion for axial pencils. 



