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2nd: The instrument of vision—the eye. Of its different parts 
the retina is the most important of the subject of the paper. Itisa 
delicate transparent membrane containing the terminal organs of the 
optic nerve. Under a powerful microscope no less than ten layers may 
be detected in a section of it. Of these the layer of rods and cones is 
the most important. It is through these rods and cones that it is supe 
posed the vibrations of the luminiferous ether are transformed into 
those molecular movements which, transmitted to the brain, give us 
the sensation of light. It isa curious fact that that portion of the 
brain which is connected by the optic nerve with the retina is incapable 
of receiving other sensation but that of light. Speaking generally our 
colour-vision is defective insomuch that there are vibrations outside 
the red and violet which do not, under ordinary circumstances, affect 
the retina. Again, there are individuals who can perfectly distinguish 
between gradations of light and shade, and yet are quite incapable of 
distinguishing not only between gradations of certain cotours, but even 
colours themselves. It is impossible to say whether Helmholz’s theory 
is the true solution of this difficult problem. 
This defect of colour vision is often congenital. It is more 
prevalent than is generally supposed. Prof. Dowden found that, out 
of 2,300 railway employés, 6°6 per cent. were colour blind. Dr. 
Jefferies, of New York, found that out of 10,387 persons 4:149 per 
cent. were so. Out of 1,000 persons personally examined the result 
was 5°3. The general average is about 4 per cent. 
Colour blindness may sometimes be due to defective education of 
the colour sense. Persons may be either unable to distinguish any 
colours, every object appearing white, black, or grey, or they may be 
blind to all red rays—the most common form—or to all green rays ; or, 
lastly, to all violet rays. A red-blind person would distinguish the 
difference between red and green only by one being brighter than the 
other. 
3rd : As to the tests by which this defect of vision may be de- 
tected. Coloured lights have been suggested, but there are objections 
to them. The method generally adopted is by a series of wools 
specially dyed for the purpose, and arranged in a certain order. 
To the green-blind, for instance, red and violet produce the sensation 
of white or grey. Purple, being composed of red and blue (or purple) 
is, to the colour blind, identical with blue. 
A red-blind person will match a bright red with a green, the only 
distinction between them being a little difference in brightness, But 
then it must be remembered that he will call the red accurately crim- 
‘son or scarlet, as the case may be, though the word he uses conveys to 
him only the impression of another shade of green. Thus, hundreds of. 
persons live and die in entire ignorance ot their peculiar want of per- 
ception, and even those with whom they are mos. familiar are as 
ignorant of this defect of vision in them as they are themselves. 
