THE: EYE AND THE PHYSIOLOGY OF VISION. 567 



line from the luminous point in question through the nodal 

 point of the eye to the retina. If the eye be a normal one 

 and focussed properly all the other rays of light from this 

 same luminous point will fall at the same point on the retina 

 as the straight unbent one passing through the nodal point. 



In Figure 178 two directions of light are indicated. 

 One, F' F" passes through the horizontal axis of the eye, 

 and passing through the point k' k" suffers no bending. 

 The line G' G" is a second direction of light passing 

 through the point k' k" and so is continued straight back 

 to the retina. 



The line of light G' G'' is the common path of light fall- 

 ing on the yellow spot. It is apparent, therefore, that the 

 geometric axis of the eye is not the path which the light 

 usually takes in entering it, but that the yellow spot is 

 slightly above the axis, thus necessitating in ordinary vision 

 a constant, although slight rotation of the eyeball upwards. 



It may be pointed out once more that all the refracting 

 media of the eye reduced to a single medium would in its 

 optical action be that of a spherical surface of vitreous 

 humor of a little over 5 mm. radius, as indicated by the 

 line // in the figure, which surface would have as its central 

 point the nodal point indicated by the letter H". 



THE LUMINOSITY OF EYES. 



On account of the black choroid which lines the inside 

 of the eye, very little light indeed is reflected back out of 

 the eye, it being all absorbed by this coat. For this reason 

 the pupil looks black. It is true, though, that even in the 

 human eye a little light is reflected out of the pupil, enabling 

 an observer by means of proper instruments to see the 

 retina. In many of the lower animals, on the other hand, 

 a very much increased amount of light is reflected through 

 the pupil, sometimes to such an extent as to make the eyes 

 luminous, as for instance, those of a dog, cat, or tiger. 

 This is brought about by the presence of a reflecting mem- 

 brane in the back of the eyes of these animals, called the 



