By PEOFESSOB EAMSAY, Ph.D. 
LL that we know of outside nature is caused by the rapid 
Xx. vibration of the nerves connected with our senses. This 
is, perhaps, most evident with the sense of hearing, for the 
vibrations are fewer in number and of larger amplitude than 
those which cause the sensations of light and heat. The lowest 
tone which can be heard vibrates at the rate of sixteen times 
a second. It is the tone of a 32-feet organ pipe. Our range 
of hearing extends for eleven or twelve octaves higher ; yet 
many ears are able to recognise a tone vibrating from 20,000 
to 40,000 times a second as a tone— a distinct musical sound — 
while others cease to hear vibrations long before their rate 
becomes so rapid. 
It has been calculated, exactly, too, what number of vibra- 
tions per second produces the impression on our retina, which 
we call colour. Thus the lowest visible red colour of the 
prismatic spectrum is caused by vibrations, 39|- trillions of which 
take place in a single second. The colours of the spectrum pass 
without break through orange-red, orange, orange-yellow, yellow, 
yellowish-green, green, greenish-blue, blue (which deepens till it 
becomes indigo blue), and then passes into violet. 
Each geometrical point of this spectrum coincides with a 
definite number of vibrations per second till we reach the violet 
end; then the most rapid vibration which we can see is 761- 
trillions per second. 
I have made measurements of the distance which various 
