138 Dr Butter m the insensihiUty of the Eye 
to the eye. The eye is but an optical instrument, serving to the 
purposes of vision ; the judgment exercised upon the visual sen- 
sations, is an after process, and. resides not in the eye. Still, 
Jiowever, the construction of the visual organ, modifies the ap- 
pearances of objects presented to it. Ail eyes do not see equally 
.well in the same light. Nevertheless, there is a standard of vi- 
sion which we call common. A difference in the vision of eyes 
depends, not unfrequently, on the colours of the iris and tape- 
lum. In Albinos, the iris is red. They cannot see distinctly 
in the day time, because the red rays of the sun are possibly re- 
flected, while the rest may be absorbed. It is probable that the 
red rays may be reflected from the iris when most closed, in Al- 
binos, because in them there is a deficiency in the pigmentum 
nigrum or black coating, which covers the choroid tunic, and 
which being wanting, allows the rays to be more reflected and 
less absorbed than they are in human eyes generally. Hence 
the pupil is almost closed in Albinos, Red, we know, strikes 
the eyes most forcibly, as it is the least refrangible colour. In 
optics, it is proved that red bodies reflect the red rays, while 
they absord the rest, and green colours reflect green rays, and 
possibly the blue and yellow but absorb the rest. Still, how- 
ever, the consciousness of colours does not depend on the colour 
of the iris, because one person having a dark iris, and another a 
light grey, can distinguish colours equally well'; nor on the ta- 
petum, by the same rule, though the use of this coloured mat- 
ter in the eye, is not yet well made out. Herbivorous animals, 
as the ox, are supposed to have the tapetum in their eyes of a 
greener colour than carnivorous animals, in order to reflect the 
green colour of the pasturage : but this explanation, given by 
Monro primus^ does not hold good, for the hare, whose tapetum 
is of a brownish chocolate, and the stag, which has a silvery blue 
tapetum inclining to a violet, is equally herbivorous with the ox. 
In man and apes, the tapetum is of a brown or blackish colour ; 
in hares, rabbits, and pigs, it is of a brownish chocolate. The 
, ox has the tapetum of a fine green-gilt colour, changing to a ce- 
lestial blue ; the horse, goat, and stag of a silvery-blue chan- 
ging to a violet ; the sheep of a pale gilt green, sometimes blue- 
ish ; the lion, cat, bear, and dolphin, have it of a yellowish-gilt 
pale ; the dog, wolf, and badger, of a pure white, bordering on 
