48 
Proceedings of the Boyal Irish Academy, 
As I conceive, the analogy lies chiefly in this — as we know, various 
solid and liquid bodies exercise a selective absorption both for heat and 
light, in virtue of which certain rays are set apart to be stopped, while 
certain others are allowed to proceed ; after an analogous fashion, 
certain nerves exercise a so-called selective power, permitting certain 
undulations to proceed, while those of a diff'erent wave length are in- 
tercepted. Most substances, including those that are transparent for 
light, are generally opaque for dark heat of great wave length and 
small refrangibility. So we have no reason to think that heat can 
excite in the retina undulations capable of being propagated by the 
optic nerve to the sensorium, although light certainly does so. 
Instead of supposing, like Dr. Brown-Sequard, that there exist a 
great number of distinct conductors, I should suppose that there are a 
great number of distinct sensations propagated along the nerve tubules, 
in undulations of different wave lengths. 
As the rays of the heat, light, and actinic spectra differ in refran- 
gibility, so do the undulations produced by heat, cold, pain, tickling, 
or the unfelt sensations (if I may use the phrase), — which last cor- 
respond to the invisible and cold actinic rays. 
As in the economy of nature the actinic rays play a part of vast 
importance, so these vibrations, which play along our nerves, without 
our knowing it, are all important in the animal economy. 
The unfelt tickling, which keeps the heart in regular and ceaseless 
action during life, is not less important to man than that part of the 
sunbeam which we cannot see, nor yet feel the warmth of, is in the 
economy of nature. 
Many phenomena such as those connected with seeing comple- 
mentary colours, when a white surface is gazed at, after the eyes have 
been fixed upon a blue, red, or yellow disc ; the phenomena, connected 
with peculiar colour, seen after the administration of santonine ; the 
effects of lead poisoning upon sensation, &c., &c., are more easily ex- 
plicable upon the undulatory than upon any other hypothesis of sen- 
sation. 
The author concluded by referring to the well-known experiments of 
Professor Tyndall, showing the power of absorption of vapours and 
scents, of which minute quantities are introduced into dry air filling 
a glass tube. In these experiments a physical change of almost incon- 
ceivable subtlety is followed by the interception of waves of radiant 
heat. So with a nerve tubule — a minute quantity, suppose, of santo- 
nine, entering into the axis cylinder of the optic nerve tubules (as the 
scent in the air filling the glass tube), intercepts some light waves of a 
certain refrangibility ; and the result is, that all objects looked upon 
have their natural colour, minus the intercepted undulations. This 
analogy serves to explain the general bearing of this hypothesis. 
