SENSE OF VISION. 879 



the Retina, on account of their minute size, and the absence of their distinctive 

 characters. According to Mr. Bowman, 1 the tubular membrane and the white 

 substance of Schwann are deficient ; and only the central part of the nerve- 

 fibre, or axis-cylinder, is continued into this expansion. The plexus of nerve- 

 fibres comes into relation with a plexus of capillary vessels (Fig. 210), very 

 minutely distributed ; and also with a layer of cells, which constitutes the in- 

 ternal layer of the true Retina, and which so closely resembles those of the 

 cortical substance of the Brain that there can be no reasonable doubt of their 

 correspondence in function. 3 We have here, then, all the elements of an ap- 

 paratus for the origination of changes in the nervous trunks, in a fully deve- 

 loped condition ; and it can scarcely be doubted 

 that the essential parts of the same structures Fig- 213. 



exist in connection with the peripheral expan- 

 sions of the nerves distributed to other sensory 

 organs. The true Retina is covered externally 

 by a very peculiar investment, the " Membrane 

 of Jacob" (Fig. 213), which separates it from 

 the pigmentary layer. This seems to be compos- 

 ed of cells having a cylindrical form ; and these 

 are sometimes arranged vertically to the surface 

 of the membrane, so that their extremities only 

 are seen j whilst in other instances they are 

 found to present an imbricated arrangement, 

 lying over each other obliquely, in which case Outer surface of the Retina, showing 



they are of considerable length (Fig. 213). the membrane of Jacob, partially detach- 



They are remarkable for the rapidity with ed> After Jaeob - 



which they undergo alterations after death, 



and especially for the changes in their form which are produced by the action 



of water. 



881. The following statements on the Limits of Human Vision, in regard to 

 the possible minuteness of the objects of which it can take cognizance, compre- 

 hend the result of numerous inquiries made by Prof. Ehrenberg, with the view 

 of calculating the ultimate power of the Microscope. 3 In opposition to the 

 generally received opinion, Ehrenberg arrived at the conclusion that, in regard 

 to the extreme limits of vision, there is little difference amongst persons of 

 ordinarily good sight, whatever may be the focal distance of their eyes. The 

 smallest square magnitude usually visible to the naked eye, either of white par- 

 ticles on a black ground, or of black upon a white or light-colored ground, is 

 about the l-405th of an inch. It is possible, by the greatest condensation of 

 light, and excitement of the attention, to recognize magnitudes between the 

 1 -405th and l-540th of an inch ; but without sharpness or certainty. Bodies 

 which are smaller than these cannot be discerned with the naked eye when 

 single } but may be seen when placed in a row. Particles which powerfully 

 reflect light, however, may be distinctly seen when not half the size of the legist 

 of the foregoing; thus, gold dust, 4 of the fineness of l-1125th of an inch, may 

 be discerned with the naked eye in common daylight. The delicacy of vision 



1 "Lectures on the Parts concerned in the Operations on the Eye," p. 81. 



2 This doctrine, which has been taught by the Author for many years, has latterly re- 

 ceived full confirmation from the researches of Mr. H. Gray upon the development of the 

 Eye ; for he has shown that the Retina is really an offset put forth (so to speak) from the 

 Optic Ganglion, and that the Optic Nerve is to be considered in the light of a commissure. 

 (See "Philos. Transact.," 1850.) 



3 Taylor's "Scientific Memoirs," vol. i. p. 576. 



4 Ehrenberg mentions that he obtained the finest particles of gold by scraping gilt 

 brass ; by filing pure gold, he always obtained much coarser particles. 



