497 
COLOUR-BLINDNESS. 
BY JABEZ HOGG, F.L.S., &C. 
T HE eye — that index of the soul, that channel of human know- 
ledge — conjures up a host of feelings when the mind is 
directed to it, as an object of especial attention. Of the five 
senses with which most of ns have been blest, the loss of 
sight seems to be the greatest calamity that can befall us. 
Reflect for a moment on the condition of those deprived of 
this exquisite gift. To what a sad state are they reduced 
who, in a perpetual darkness in the midst of light, have not 
anything like a conception of what we mean when we talk of 
the golden sun, the bright stars, the ever-varying tinted 
flowers, the beauty of spring, the glow of summer fields, the 
ripening fruits of autumn, and all beside that clothes the face 
of nature so beauteously to our eyes ! 
Our theme, however, is not with those who have so large a 
claim to our sympathies, but rather with others among us who 
suffer from a partial kind of blindness, — not necessarily a 
mechanical or optical defect, but one which is almost unknown 
or unrecognized by those who suffer from it, and, being- 
ignorant of its existence themselves, cannot easily be per- 
suaded to believe it. 
An explanation of this curious defect will be worth while 
listening to, the more so as many eminent philosophers have 
suffered from it ; and it is perhaps owing to this circumstance 
that so much time and attention has been given to the inves- 
tigation of so curious an anomaly. It is well known that a ray 
of light, from any source, may be divided by means of a prism 
into a number of rays of different refrangibility, forming a 
series, and called a spectrum, the most familiar instance of 
which is the rainbow. The drops of rain falling between the 
sun and the eye act as so many prisms, and each ray is 
thereby bent or refracted to a different angle, the red most and 
the blue least ; and as thus the rays of light are made to enter 
the eye separately, we have produced the beautiful prismatic 
phenomenon of the rainbow, the outermost colour of which is 
red, the innermost violet, and the intermediate, from slightly 
intermixing and overlapping each other, we respectively name 
orange, yellow, green, blue, and indigo. The three homo- 
geneous colours — yellow, red, and blue — have been shown by 
