246 



OF SENSATION, AND THE ORGANS OF THE SENSES. 



[Fig. 47. 



the Frog, the diameter of the ultimate nervous 

 fibres is stated by Treviranus at about y^Q-th 

 of an inch ; whilst that of the papillae is about 

 i th of an inch. In Birds and Mammalia, 







A portion of the Retina of an Infant, 

 with its vessels injected and magnified 

 25 diameters. An outline of the natural 

 size of this piece is seen just below the 

 main cut.] 



however, the papillas, as well as the nervous 

 fibrils, are much smaller; in the former the 

 diameter of the papillae is stated at from about 

 __^.__th to fi - 3 J 5o tn f an inch; in the Rabbit 

 at yYQ-oth of an inch ; and in Man at from the 

 ^^--th to grV^h f an inch.* An attempt 

 has been made to show, that the size of the 

 papillae determines that of the smallest object, 

 which can be seen by the unaided eye ; and 

 it is a curious fact, that the calculation long 

 ago made by Smith, in regard to the size of 

 the most minute sensitive point upon the retina, 

 founded upon the dimensions which the image 

 of the minutest visible object will possess, co- 

 incides exactly with the measurement of Weber. 

 There is no doubt, however, that, under favour- 

 able circumstances, the eye will take cognizance 

 of objects much smaller than those on which 

 Smith's calculation was founded. The follow- 

 ing statements on this interesting subject com- 

 prehend the result of numerous inquiries re- 

 cently made by Ehrenberg, with the view of 

 establishing the limits of Human Vision, as a 

 datum from which to calculate the ultimate power of the Microscope.! 



333. 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 particles on a black ground, or of black upon a 

 white or light-coloured ground, is about the 4 ^th of an inch. It is possible, by 

 the greatest condensation of light, and excitement of the attention, to recognize 

 magnitudes between the j^-jth and j^th 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. Parti- 

 cles which powerfully reflect light, however, may be distinctly seen, when not 

 half the size of the least of the foregoing ; thus, gold dustf of the fineness of 

 of an inch, may be discerned with the naked eye in common daylight, 

 e delicacy of vision is far greater for lines than for single articles ; opaque 

 threads of T -oVo- tn f an inch in diameter may be discerned with the naked 

 eye, when held towards the light. Such threads are about half the diameter 

 of the Silkworm's fibre. It is evident, from these facts, that the images of 

 such particles formed upon the retina, must be considerably smaller than the 

 diameter of the papillae. Still it is by no means improbable that, when we are 

 looking at a continuous surface, the diameter of the papillae will regulate our 



* This is the diameter assigned by Weber to what he terms the globules of the Retina; 

 there can be little doubt, however, that these are identical with the papillas, since the 

 latter are very apt to separate, in eyes which are examined even a short time after death, 

 from the fibres beneath. 



j- Taylor's Scientific Memoirs, vol. i. p. 576. 



t Ehrenberg mentions that he obtained the finest particles of gold, by scraping gilt brass ; 

 by filing pure gold he always obtained much coarser particles. 



