126 ANIMAL MECHANISM. 



there must be surface to create an image on, and by the 

 laws of optics, the representation of the object, without an 

 additional glass within the eye, must necessarily be as 



HOW THE OBJECT APPEARS IN ITS TRUE POSITION, THE 

 IMAGE BEING INVERTED. 



Habit is supposed to be the cause of seeing objects as 

 they really exist, in relation to surrounding bodies.* A 

 few philosophers conceive that the mind contemplates 

 the object only, without reference to its representative 

 on the retina, which is made there as a natural result. 

 Certain it is, that without the image, there is no vision. 

 How the brain is operated upon by the light that defines 

 the object, will probably never be known. The minute- 

 ness of the miniature traced on the retina, precisely like 

 the object in every minute particular, is truly astonishing. 

 By cutting off the coats of a bullock's eye, and holding 

 a clean white paper near, this beautiful exhibition can be 

 leisurely observed. If a sheet of white cotton cloth, six 

 feet square, is elevated '24,000 feet in the air, the eye 

 being supposed one inch in diameter, the miniature of 

 the sail on the retina will be only one eight thousandth 

 part of an inch square ; which is equivalent to the 666th 

 part of a line, being only the 6(ith part of the width 

 of a common hair ! Leaving this point to philosophers, 

 we proceed to such facts as are susceptible of positive 

 demonstration. 



HOW, WITH BOTH EYES, ONLY ONE OBJECT IS SEEN. 



At one side of the centre of each eye, there is a sur- 

 face more susceptible of visual impressions than any 

 other. These points correspond in both eyes being 

 precisely on the two retinas alike. An impression there- 

 fore on one, provided the light strikes them equally, pro- 

 duces precisely the same effect on both. This, instead 



* A Dr Reed, of Cork, has recently attempted to demonstrate 

 that the cornea is the true seat of vision, and that we see by means 

 of erect and reflected, and not by refracted and inverted images. 



