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



touch food until it is far advanced in putridity, so do we find that men who 

 are compelled by circumstances to live upon putresceut food, come at last to 

 relish it most when it is furthest advanced in decomposition ( 71); and the 

 most refined epicures among highly civilized communities seem to find pleas- 

 ure in similar odors and savors, which to ordinary tastes are anything but 

 agreeable. 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 themselves have been completely re- 

 moved from the nasal passages, since it is not unlikely that the odorous par- 

 ticles (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 many days after a post-mortem examination. 1 



5. Sense of Vision. 



60S. The objects of this sense are bodies from which Light proceeds either 

 because they are luminous in themselves, or because they reflect the light 

 that proceeds from other bodies. Whether this 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 Physiol- 

 ogist ivhich is the true solution, since he is only concerned with the laws ac- 

 cording to which the transmission takes place, which are the same on both 

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



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

 diverge in every direction, so as to form (as it were) 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, toivards the perpen- 

 dicular 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 oppo- 

 site 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 consequently greater. The rays diverging from the several points 

 of a luminous object, are thus brought to corresponding foci; and the places 

 of all these foci hold exactly the same relation to each other, with that of the 

 points from which the rays diverged; so that a perfect image of the object is 

 formed upon a screen held in the focus of the lens. This image, however, 

 will l)e inverted; and its size, in proportion to that of the object, will de- 

 pend upon their respective distances from the lens. If their distances be the 

 same, their size will also be the same; if the object be distant, aud the image 

 in :ir, the latter will be much the smaller; and vice 



1 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. For some sensible 

 observations on the sense of smell in the diagnosis of disease, see D. W. Cromptmi, in 

 the Birmingham Medical Review, No. viii, Oct. 1873, p. 251, and No. ix, p. l':J. 



