The Senses of Animals. 275 



certain intermediate ganglia, to the delicate transparent 

 nerve-fibres in the front of the retina. These collect to a 

 certain spot, where they pass through the retina to form the 

 optic nerve. Where they pass through the retina there can, 

 of course, be no rods and cones. And in this spot there is 

 no power of vision. It is the blind spot. The reality of its 

 existence can easily be proved. Make a dot on a piece of 

 writing-paper, and about three inches to the left of it place 

 a threepenny or sixpenny bit. Close the right eye, and look 

 with the left eye at the dot. The sixpenny bit will also be 

 seen, but not distinctly. Keep the eye fixed on the dot, 

 and move the head slowly away from the paper. At a 

 distance of about ten inches the coin will completely dis- 

 appear from view. Its image then falls on the blind spot. 



The organ of vision, then, in us consists of an essential 

 sensory membrane, the retina, with its delicate rods and 

 cones ; and an accessory apparatus for focussing an inverted 

 image on to the sensitive surface of the retina. The 

 surface is not, however, equally sensitive, or, in any case, 

 does not give an equal power of discrimination, throughout 

 its whole extent. This is seen in the experiment above 

 described. When we look at the dot we see the coin, but 

 not distinctly. The area of clear and distinct vision is, in 

 fact, very small, constituting the yellow spot about ^ of 

 an inch (2 millimetres) long, and 3^ of an inch ('8 milli- 

 metre) broad. And even within this small area there is a 

 still more restricted area of most acute sensibility only T ^Q 

 of an inch (-2 millimetre) in diameter. Nevertheless, 

 within this minute area there are some two thousand 

 cones, the rods being here absent. In carefully examining 

 an object we allow this area of acute vision to range over 

 it. Hence the extreme value of that delicate mobility 

 which the eye possesses a mobility that is accompanied 

 by muscular sensations of great nicety. 



We saw that the sense of touch in the tongue is 

 sufficiently delicate to enable us to recognize, as two, 

 points of contact separated by ^5 of an inch (I'l milli- 

 metre). What, in similar terms, is the delicacy of sight ? 



