56 INNEKVATION. [cHAP. XVII. 



fibrous internal surface with the axes of the tubules of the optic nerve, 

 and having its external surface formed by a structure similar to that of 

 the cineritious substance of the cerebral hemispheres. Its per- 

 meation by a close network of capillaries assimilates it still further 

 to the gray nervous matter; for which reasons it may be con- 

 sidered as a portion of the cerebrum advanced towards the surface 

 of the body into a suitable relation to a dioptric apparatus for the 

 reception of rays of light from external objects. The optic nerve 

 may be regarded as a commissure between the gray nervous sheet 

 within the sclerotic, and the gray nervous matter of certain parts 

 of the cerebrum. We have no more reason to deny the immediate 

 connexion of the sensorium with the retina, than its immediate 

 connexion with any small portion of the cerebral convolutions, 

 duly united with the rest. The nature of the connexions between 

 the retina and the brain, and the phenomena to which their dis- 

 ruption gives rise, have been the occasion of many interesting 

 speculations regarding the mode in which the mind is reached, 

 or, in other words, as to how an impression on the retina becomes 

 a sensation to the mind. But we shall probably be disappointed if 

 we imagine that any facts which have been or may be hereafter 

 ascertained, are capable of leading to the solution of a problem 

 too inscrutable for our limited powers. 



It is a matter of considerable interest, as regards the mode of 

 action of the retina in vision, to determine how distant the images 

 of two points on the retina must be, to be seen distinctly as two ; 

 in other words, how small a portion of the retina is capable of 

 independent sensation. As the result of many experiments and 

 calculations by Smith, Harris, and others, this may be stated as 

 probably about 80 * 00 of an inch, so that the objects must subtend 

 an angle of at 1 least 40". Two points within an angle of that size 

 would appear as one. It is a question somewhat different, what 

 is the smallest portion of the retina capable of sensation; and 

 undoubtedly an object whose sides subtend an angle very much 

 smaller than the preceding may be visible, if sufficiently bright. 

 But this circumstance of quantity of light introduces a difficulty 

 into the inquiry, since even a mathematical point, if sufficiently 

 brilliant and out of focus, might become visible by its circle of 

 aberration on the retina; although, if its rays met in the retina, it 

 would be invisible. But, in carrying our speculations thus far, we 

 must cease to regard the retina as a mathematical plane, and re- 

 member that it has a certain thickness, in traversing which the rays 

 would necessarily cover more than a point, either in front of or 



