ON SMELL. 
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eyes, belonging to about 160 individuals, have the power of 
seeing — in other words, their limit of vision — and in most 
instances there is great regularity. Some colour-blind persons, 
however, had the power of vision prolonged more than usually 
far into the violet, while one or two persons with no dis- 
coverable peculiarity of vision, were able to see colour beyond 
the usual limit at the red end of the spectrum. Although 
sound is caused by comparatively slow vibrations of air, or 
other gas or liquid, successive waves of which impinge on 
the tympanum or drum of the ear, yet there can be little doubt 
this vibration is communicated to the aural nerve, in which it 
produces some electrical and chemical changes, rapidly alter- 
nating with the original condition of the nerve, and so the 
sensation passes the boundary-line of our power to observe it, 
for it then ceases to be physical, and enters the domain of 
mental phenomena. Similarly, light produces certain rapidly 
successive changes in the retina and optic nerve, and is thus 
conveyed as an impression to the brain. 
It may be objected that the causes of impressions are 
different : that air, a weighable substance, and therefore matter, 
produces the sensation of sound : that the vibrations are like 
those of a vibrating spiral spring when stretched and released 
in the direction of motion : whereas light is caused by the trans- 
verse vibrations, comparable to those caused by the circles of a 
stone thrown into water, of a hypothetical substance called ether ; 
and that the difference in rate is so prodigious as to preclude 
comparison. But, I answer that these vibrations are soon 
communicated to the optic or aural nerve, and are then probably 
a series of electrical or chemical changes, recurring a certain 
number of times a second; and that we are ignorant of the 
precise nature of such vibrations, whether they are longitudinal 
or transverse. 
As yet we have been considering the nature of two only of 
