SENSE OF VISION. 



413 



separates it from the pigmentary layer. This seems to be composed of cells 

 having a cylindrical form. These are sometimes arranged vertically to the 



[Fig. 178. 



as* 



& W 



771 



i 'in 



Vertical section of the Human Retina and Hyaloid Membrane, h. Hyaloid membrane, h'. Nuclei 

 on its inner surface, c. Layer of transparent cells, connecting the hyaloid and retina, c'. Separate 

 cell enlarged by imbibition of water, n. Gray nervous layer, with its capillaries. 1. Its fibrous lamina. 

 2. Its vesicular lamina. 1'. Shred of fibrous lamina detached. 2'. Vesicle and nucleus detached, g. Granu- 

 lar layer. 3. Light lamina frequently seen. g'. Detached nucleated particle of the granular layer. 

 m. Jacob's membrane, in'. Appearance of its particles, when detached, m" . Its outer surface. Mag- 

 nified 320 diameters.] 



surface of the membrane, so that their extremities only are seen ; whilst in 

 other instances they are found to present an 

 imbricated arrangement, lying over each other 

 obliquely, in which case they are of conside- 

 rable length (Fig. 175). They are remarka- 

 ble for the rapidity with which they undergo 

 alterations after death ; and especially for the 

 changes in their form, which are produced 

 by the action of water. 



540. The following statements on the 

 Limits of Human Vision, in regard to the 

 possible minuteness of the objects of which 

 it can take cognizance, comprehend the re- 

 sult of numerous inquiries made by Ehren- 

 berg, with the view of calculating the ulti- 

 mate power of the Microscope.* In opposi- 

 tion to the generally-received opinion, Ehren- 

 berg 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 

 particles on a black ground, or of black upon a white or light-coloured 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 

 l-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 



Outer surface of the Retina, showing 

 the membrane of Jacob, partially detach- 

 ed. After Jacob.] 



Taylor's Scientific Memoirs. 

 35* 



Vol. i. p. 576. 



