SENSE OP VISION. 873 



was possessed of a sensibility so acute in this respect, that he perceived the 

 proximity of one that had been accidentally shut up in a closet adjoining his 

 room. Among Savage tribes, whose senses are more cultivated than those of 

 civilized nations, more direct use being made of the powers of observation, the 

 scent is almost as acute as in the lower Mammalia ; it is asserted by Humboldt, 

 that the Peruvian Indians in the middle of the night can thus distinguish the 

 different races, whether European, American Indian, or Negro. 1 The agreeable 

 or disagreeable character assigned to particular odors is by no means constant 

 amongst different individuals. Many of the lower Animals pass their whole 

 lives in the midst of odors, which are to Man (in his civilized condition at least) 

 in the highest degree revolting ; and will even refuse to touch food until it is far 

 advanced in putridity. It more frequently happens in regard to odors and savors, 

 than with respect to other sensory impressions, that habit makes that agreeable, 

 and even strongly relished, which was at first avoided ; the taste of the epicure 

 for game that has acquired ihefumet for olives for assafoetida, &c., are in- 

 stances of this. As to the length of time during which impressions made upon 

 the organ of smell remain upon it, no certain knowledge can be obtained. It 

 is difficult to say when the effluvia have been completely removed from the nasal 

 passages, since it is not unlikely that the odorous particles (supposing such to 

 exist) are absorbed or dissolved by the mucous secretion ; it is probably in this 

 manner that we may account for the fact, well known to every medical man, 

 that the cadaverous odor is frequently experienced for days after a post-mortem 

 examination. 3 



5. Sense of Vision. 



874. The objects of this sense are bodies, which are either in themselves 

 luminous, or which become so by reflecting the light that proceeds from others. 

 Whether their light is transmitted by the actual emission of luminous particles, 

 or by the propagation of undulations analogous to those of sound, is a question 

 that has been long keenly debated amongst Natural Philosophers ; but it is of 

 little consequence to the Physiologist which is the true solution, since it is only 

 with the laws according to which the transmission takes place that he is con- 

 cerned. These laws it may be desirable here briefly to recapitulate. 



875. Every point of a luminous body sends off a number of rays, which 

 diverge in every direction, so as to form a cone, of which the luminous point is 

 the apex. So long as these rays pass through a medium of the same density, 

 they proceed in straight lines ; but, if they enter a medium of different density, 

 they are refracted or bent towards the perpendicular to the surface at the 

 point at which they enter, if they pass from a rarer into a denser medium, and 

 from the perpendicular, when they pass from a denser medium into a rarer. It 

 is easily shown to be a result of this law, that, when parallel rays passing through 

 air fall upon a convex surface of glass, they will be made to converge ; so as to 

 meet at the opposite extremity of the diameter of the circle, of which the curve 

 forms part. If, instead of continuing in the glass, they pass out again, through 

 a second convex surface, of which the direction is the reverse of the first, they 

 will be made to converge still more, so as to meet in the centre of curvature. 

 Rays which are not parallel, but which are diverging from a focus, are likewise 

 made to converge to a point or focus ; but this point will be more distant from 

 the lens, in proportion as the object is nearer to it, and the angle of divergence 



1 The Author has been assured by a competent witness, that a youth in the state of, 

 Hypnotism had his sense of Smell so remarkably heightened, as to be able to assign (with- 

 out the least hesitation) a glove placed in his hand, to its right owner in the midst of 

 about thirty persons, the boy himself being blindfolded. 



2 This may partly be attributed also to the effluvia adhering to the dress. It has been 

 remarked that dark cloths retain these more strongly than light. 



