[From the Report of the British Association, 1870.] 



XLII. On Colour-vision at different points of the Retina. 



IT haa long been known that near that point of the retina where it is 

 intersected by the axis of the eye there is a yellowish spot, the existence of 

 which can be shewn not only by the ophthalmoscope, but by its effect on 

 vision. At the Cheltenham Meeting in 1856 the author pointed out a method 

 of seeing this spot by looking at that part of a very narrow spectrum which 

 lies near the line F. Since that time the spot has been described by Helmholtz 

 and others ; and the author has made a number of experiments, not yet pub- 

 lished, in order to determine its effects on colour-vision. 



One of the simplest methods of seeing the spot was suggested to the 

 author by Prof. Stokes. It consists in looking at a white surface, such as that 

 of a white cloud, through a solution of chloride of chromium made so weak that 

 it appears of a bluish-green colour. If the observer directs his attention to 

 what he sees before him before his eyes have got accustomed to the new tone of 

 colour, he sees a pinkish spot like a wafer on a bluish-green ground; and this 

 spot is always at the place he is looking at. The solution transmits the red 

 end of the spectrum, and also a portion of bluish-green light near the line F. 

 The latter portion is partially absorbed by the spot, so that the red light has 

 the preponderance. 



Experiments of a more accurate kind were made with an instrument the 

 original conception of which is due to Sir Isaac Newton, and is described in 

 his Lectiones Opticce, though it does not appear to have been actually constructed 

 till the author set it up in 1862, with a solid frame and careful adjustments. 

 It consists of two parts, side by side. In the first part, white light is dis- 

 persed by a prism so as to form a spectrum. Certain portions of this spectrum 

 are selected by being allowed to pass through slits in a screen. These selected 

 portions are made to converge on a second prism, which unites them into a 



