758 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



black upon a white or light-colored ground, is about the ^th of an inch. 

 It is possible, by the greatest condensation of light, and excitement of the 

 attention, to recognize magnitudes between the ^-jth and ^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. Particles which powerfully reflect light, however, may be 

 distinctly seen, when not half the size of the least of the foregoing ; thus 

 gold-dust 1 of the fineness of yjV^th of an inch, may be discerned with the 

 naked eye in common daylight. The delicacy of vision is far greater for 

 lines than for mere points; since opaque threads of 4^Ya tn of an inch in 

 diameter (about half the diameter of the Silkworm's fibre) may be discerned 

 with the naked eye, when held towards the light. The size of the retinal 

 image in these cases must be exceedingly small. In some of Bergmanu's 

 experiments it was found that black and white checkers of ^th of an inch 

 square could be discerned at such a distance that the retinal" image of each 





FIG. 270. 



Diagrammatic section through the macula lutea and fovea centrnlis of the Retina of Man. 1. Optic 

 nerve-fibres. 2. Ganglion-cells. 3. Internal granulated (molecular) layer and Internal granule-layer. 

 4. External granulated (molecular) layer, External flhrous layer, and External granule-layer. 5. Rods 

 and cones. 6. Pigment layer. 



square could not have exceeded half the diameter of one of the cones of 

 the bacillary layer. 2 The degree in which the uttrnti'on is directed to them, 

 has a great influence on the readiness with which very minute objects can 

 be perceived ; and Ehrenberg remarks that there is much greater difference 

 amongst individuals in this respect, than there is in regard to the absolute 

 limits of vision. Many persons can distinctly see such objects, when their 

 situation is exactly pointed out to them, who cannot otherwise distinguish 

 them ; and the same is the case with persons of acuter perception, with re- 

 spect to objects at distances greater than those at which they can see most 

 clearly. "I myself," says Ehrenberg, "cannot see 27 1 th of an inch, black 

 or white, at twelve inches' distance; but having found it at four or five 

 inches' distance, I can remove it to twelve inches, and still see the object 

 plainly.' ^ Similar phenomena are well known in regard to a balloon or a 

 faint star in a clear sky, or a ship in the horizon : we easily see them after 

 they have Keen pointed out to us; but the faculty of readily descrying ob- 

 jects depends on the habit of using the eyes in search of them, and of at- 



1 Khrenberg mentions that he obtained the finest particles of gold by scraping gilt 

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

 1 llcnle and Meissner's Uericht, 1857, p. 559. 



