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ORGANS OF PERCEPTION. 191 
accurately converge upon the retina, do not produce a dis- 
tinct image. 
The power of seeing objects beyond or within the ordi- 
nary limit of distinct vision, is greatly strengthened by ha- 
bit. Thus, a sailor will discern the masts of a vessel ap- 
pearing at a distance in the horizon, where nothing is visi- 
ble to the eye of alandsman. In like manner, a botanist 
will detect a Lecidea on a rock, or the entomologist a fly, 
where an ordinary observer would perceive no trace of or- 
ganized existence. 
Some animals are destined to perform the functions of 
vision in the full light of day, while others are confined to 
the obscure light of the evening or night. In the animals 
of the former class, termed diwrnal, the mucous pigment 
of the eye is of a dark colour. The purpose which it is 
supposed to serve, is that of absorbing the rays of light, 
after passing through the retina, and of preventing any 
reflection of the rays taking place in the lateral parts of 
the eye, and disturbing the image of the objects so con- 
templated. Where the tapetum exists, however, a por- 
tion of the rays must be reflected ; but the reflection in 
this case may be so regulated, as to assist rather than dis- 
turb the action of the retina. In animals which seek their 
food in the dark, the eye is usually of a large size; the 
pupil is wide, to admit a greater number of rays; and the 
pigment and tapetum pale coloured, approaching to white. 
In these animals, as the cat, for example, whose eyes are 
so constructed, that the choroides reflects, instead of absorbs 
the rays of light, it is difficult to determine whether the 
reflected rays act upon the retina, and excite vision in their 
passage outwards, as is generally supposed; or pass through 
the retina outwards, without exciting any action, to be 
thrown on the object, in order to increase the distinctness 
of its image, by an increase of its light. It is not, indeed, 
