AND THE MICROSCOPE. 23 



message at a time, and therein they resemble simple-witted 

 servants. Two notices given to them simultaneously, become 

 blended into a single one. This is most readily exhibited by 

 touching with the points of an open pair of compasses, 

 those parts of the body where the nerves are much 

 isolated and lie far apart, as in the upper arm or the 

 median line of the back. When the points are as much as 

 an inch apart, one single prick only is felt on the places 

 mentioned, because the nerves are so distant from each 

 other that the two pricks fall in the district of one fibre, 

 and this is incapable of conveying away more than one 

 impression at a time. 



After these general explanations of the peculiar nature of 

 nervous action, we may again approach our proper subject, 

 by the special consideration of the optic nerve. Where this 

 enters the eye-ball, it consists of a tolerably thick bundle 

 composed of numerous distinct nervous fibres, and these 

 expand in the eye-ball into a hemispherical plate, in such a 

 manner that every fibre occupies a small portion of this 

 plate. The eye-ball itself, however, perfectly resembles 

 an optical instrument, a camera obscura, and the hemis- 

 pherical layer of the optic nerve, called the retina, corres- 

 ponds to the sheet of paper which receives the picture. 

 Every one of the fibrils on which the picture falls, imme- 

 diately catches up a point of it, and brings an account 

 thereof to the brain where the conceiving soul has its 

 abode, and then this has to construe the picture out of all 

 these separate points. Whether this is truly or falsely 

 construed depends on the training and cultivation which the 

 mind has received. I may be told that we have not the 

 slightest consciousness of this construction, and that sight 

 must be a much more simple matter than this. However, 

 we can easily find some examples, showing that it is only 



