82 COLOUR VISION 



the fovea it is much less than for the periphery. The point is one 

 having a very practical bearing and some experiments made by Gotch^ 

 may be quoted. 



" If the eye is fixed in the dark room on one small spot of light (A) 

 and a second small coloured light (B) is flashed out at some little distance, 

 then the sensation produced by A varies with the position and hue of B. 



"KB is spectral green or blue, and is some degrees away from A 

 (A being fixed by the visual gaze and thus in the centre of the field of 

 vision), B gives rise to a sensation of rather dazzling white. This is 

 especially the case when the light itself is of very small angular area. 

 The sensation continues when B is moved nearer to A, and tends to mask 

 any recognition of green or blue colour. In my own case such colour 

 recognition may not occur with the Board of Trade light green light 

 until this is focussed on the retina within 1|° to 2° from the centre when 

 the dazzle disappears and the colour becomes quite plain. 



" If B is red, then neither the colour nor the light itself is recognised 

 when it is so situated as to be focussed on the retina over 8° or 10° 

 outside A. In my own case with a dim Board of Trade light red, the 

 limit of failure of colour recognition was a little under 6|°. Beyond 

 such recognition limiting distance there was no sensation of white 

 dazzle, but a barely perceptible dull grey spot was sometimes seen 

 instead of the red light. Further out this light became quite invisible. 

 When brought as near as 6° it always showed as a distinct red spot, 

 and continued so however close it was brought. There was no simul- 

 taneous white dazzle to mask the hue ; thus the light, if visible was 

 easily recognised. 



" A number of experiments on individuals ascertained to possess 

 normal colour vision support the view that, in the dark-adapted eye 

 red light is recognised as red over an area whose radius is three or four 

 times that observed with green light ; yet the red light is not seen at 

 all outside this larger area. On the other hand green (or blue) light, 

 whilst it is only recognisable as green over the much more restricted 

 central area, is seen as a bright light of a dazzling white type over a very 

 extensive area. 



" As illustrations of such visual phenomena in connection with what 

 is termed ' dark adaptation perimetry,' I append a few experimental 

 results obtained from ten different observers, all with normal colour 

 vision. 



^ Report of Departmental Committee on Sight Tests, Appendix 3, 152. 1912. 



