SENSE OF HEARING THEORY OF ACOUSTICS. 779 



that, iu ordinary Vision, the immediate object of our sensation is a certain 

 condition of the retina, which is excited by the formation of a luminous 



image. 



637. The visual power is susceptible of extraordinary improvement, through 

 the habitual direction of our attention to the effects produced upon our con- 

 sciousness by the impressions transmitted to the Sensorium from the Eye ; 

 and this improvement may take place, either iu regard to the quickness and 

 readiness with which objects generally are perceived, or in the faculty of 

 discriminating the slightest differences in form, shade, color, etc., or of dis- 

 cerning bodies of extreme minuteness. In regard to all these points it may be 

 noticed that the habit of attention to any particular class of objects, sharpens 

 the discriminating power for that class alone ; and that it is usually rather 

 the mental than the corporeal vision which undergoes improvement. Thus 

 the seaman who makes out the "loom of the land," where the landsman can 

 discern nothing but an indefinite haze above the horizon, or who can distin- 

 guish the size, rig, and course of a vessel, which the landsman can but just 

 see as a formless speck, does so in virtue of the aptitude of his mind for 

 receiving suggestions from minute indications such as pass unnoticed by 

 those who have not been accustomed to form their ideas upon the same kind 

 of experiences. And the Microscopist, who is constantly on the outlook for 

 the various forms of organic structure with which his mind is familiar, dis- 

 cerns these without difficulty or hesitation, where an ordinary observer sees 

 nothing but a confused jumble of tissue. Extremely slight variations in the 

 relative illumination of two objects can readily be discerned. According to 

 Arago 1 the difference can be perceived when it amounts to no more than 

 about g'jth, according to Volkmann when it is from g' th to y^gth, to Stein- 

 heil ^gth, and to Masson, yAoth. Aubert 2 has shown that these variations 

 in the results obtained by different observers are probably due to their 

 having employed different amounts of illumination ; since the perception of 

 slight variations is much greater, within certain limits, with moderately 

 bright than with feeble illuminating powers. It is interesting to observe 

 that the power of descrying objects at vast distances appears to be heredi- 

 tarily possessed by two races of men, the Mongols of Northern Asia, and the 

 Hottentots of Southern Africa, both of which habitually dwell on vast 

 plains that seem to stretch without limit in every direction. It seems prob- 

 able that this power was in the first instance acquired by habit in every case; 

 and that, as frequently happens with acquired peculiarities which are kept 

 up by constant use in successive generations, 3 it has become fixedly he- 

 reditary. 



6. Sense of Hearing. 



638. In the Ear, as in the Eye, the impressions made upon the sensory 

 nerve are not at once produced by the body which originates the sensation ; 

 but they are propagated to it, through a medium capable of transmitting 

 them. We obviously take cognizance by the mind, therefore, not of the 

 sonorous object, but of the condition of the auditory nerve ; and all the 

 ideas we form of sounds, as to their nature, intensity, direction, etc., must be 

 based upon the changes which they produce in it. The complex contriv- 

 ances which we meet with in the organ of Hearing among higher animals, 

 are evidently intended to give them greater power of discriminating sounds 

 than is possessed by the lower tribes ; in which last it is reduced to a form 



1 Astronomie, vol. i, p. 194 * Physiologic der Netzhaut, 1864. 



3 See Princ. of Comp. PLysiol., \ 620. 



