9S Mr. Mayo on Human Physiology. 



of the retina, it appears double, and the direction to which 

 it is referred has a determinate relation to the place on 

 which its image falls. 



2. If the head of a pin strongly illuminated be viewed at 

 the distance of eighteen inches, through a card perforated 

 with two pinholes, it appears not double but single ; and 

 its apparent place does not shift on closing one or other of 

 the pinholes. 



In other words, the direction in which rays of light fall 

 upon the retina has no influence in determining the di- 

 rection in which we see the given point of an object ; or it 

 is a law in the constitution of the eye, that we refer all im- 

 pressions made upon a given point upon the retina to a 

 visual object seen in one and the same direction ; that di- 

 rection appears to be a line vertical to the point of the 

 retina, on which the impression is made. Thus, when we 

 produce an appearance of coloured circles, by pressure 

 upon the retina above, below, internally, or at the outer 

 part, the spectrum in succession shifts its place, being al- 

 ways seen in a direction exactly opposite or vertical to 

 the point where the pressure is made. 



Education, therefore, which teaches us the real magnitude 

 and true distance of objects, has nothing to do with forming 

 our notions of their true direction from us, or of their 

 visual magnitude. 



With reference to the second point adverted to, Mr. Mayo 

 remarks, *' that the readiest manner of accounting for the 

 alteration of the size of the pupil is to suppose the sub- 

 stance which forms the unattached margin of the iris irri- 

 table. In many instances the iris distinctly consists of two 

 portions, which appear from their colour to be differently 

 organized, of an outer broader part, and an inner narrow 

 ring. In birds especially, in which the pupil is as mobile 

 as their vision is perfect, the inner ring of the iris generally 

 presents a hue totally different from the outer, beginning 

 at an abrupt line. In the ring-necked parroquet, the inner 

 ring of the iris, which has a grey colour, becomes sensibly 

 narrower when the pupil contracts. 



'* Upon watching the eye of a cat or hawk, the contraction 

 of the pupil appears often to be voluntary. When the eye 

 of the animal is bent upon an object that excites its attention, 

 yet which does not shift its position, the pupil may be seen 

 to enlarge and contract alternately. The animal is pro- 

 bably employed in examining the object under different 

 lights, by intentionally admitting more or fewer rays through 

 the pupil. 



