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 a landsman. 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 diurnal, 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 ray ^v,J 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, wit hout 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, 



