310 Report of the Committee on Colour- Vision. 



greater precision when pictured on this area than when pictured 

 on any other part of the retina. Colour-vision becomes pro- 

 gressively less and less acute from the centre to the periphery 

 of the retina. 



" Hence, in considering defects of vision, it is important to dis- 

 tinguish between those which affect the centre of the field and 

 those which affect the more peripheral parts. A defect involving 

 the centre implies an impairment of colour-sense and of form- 

 sense at the point where they are most acute. 



" The form-sense at the macula lutea is tested by ascertaining 

 what is the smallest type which can be read at a given dis- 

 tance — according to the principle laid down by Prof. Snellen. 

 (Snellen's Test-types.) 



" The colour sense at the macula is tested by holding a small 

 coloured object on the end of a black wire or rod at a convenient 

 distance in front of the patient, and moving it in such a way that 

 its image moves across his retina from the periphery to the 

 centre. If there is a defect at the macula the colour, instead of 

 appearing most intense at that part, appears less intense, or is 

 lost altogether. I commonly employ a circular piece of red 

 sealing wax on the end of a wire. 1 make the patient stand with 

 his back to a window, cover one eye with his hand, and look 

 straight at my forehead with the other. Watching that he does 

 not move his eye, I hold the red object before him at 30 or 

 40 degrees to one side of his line of vision. I ask him the colour. 

 He says ' red.' I try again at the other side, and above, and 

 below the line of vision, with the same result. I then move the 

 object into his line of vision, and repeat the question. If his 

 vision is impaired at the macula he says ' it looks brown,' 

 or i dull,' or ' dirty,' or ' I can't see it at all.' He has a 

 ' central scotoma ' : a central area of defective vision — 

 an ' absolute scotoma ' if vision is entirely lost in this area ; 

 a ' colour scotoma ' if the object is still perceived, but not its 

 colour. A saturated colour gives the clearest indications ; a pale 

 colour is a more delicate test for slight defects, but requires 

 better power of observation on the part of the patient. Red is 

 practically the most effective test. When red is lost, green 

 is lost also. Green is said to be lost before red. I cannot speak 

 positively of this from my own observation. In order to test this 

 point it would be necessary to choose a green and a red of pre- 

 cisely equal intensity — i.e., of equal white-value. 



" Central Scotoma is caused by various affections of the optic 

 nerve, the choroid, and the retina. I exhibit charts taken from 

 three cases of the kind, which show the position and extent of 

 the affected area. 



" Central Colour Scotoma due to excessive use of Tobctcco, is one 

 of the commonest forms. I hand in some statistics which show 

 that this condition — known as tobacco amblyopia— constitutes 

 rather more that 1 per cent, of eye disorders in my own hospital 

 practice ; rather less that 1 per cent, in my private practice. 



