Thus, if red rays encounter certain tactile end-organs in the skin, they are perceived by the mind as heat, and if they pass into the eye and stimulate the retina, they are perceived as red light. In other words, in whatsoever manner an end-organ is stimulated, it only induces its own particular sensation. How it comes about that the various end-organs induce such different sensations is not yet known. The ultra-violet theory of olfaction, however, has to run the gauntlet of much more serious criticism than the difficulty we have just disposed of. One great objection to it (to my mind) is that it fails to account for another absorption phenomenon of which I have not yet made any mention. It was first observed by Tyndall nearly fifty years ago. On submitting odorous vapours to examination Tyndall found, not that they absorbed ultra-violet rays, as this method is of quite recent usage, but that they _absorbed heat-rays_, or the _infra-red rays_ of the spectrum. So that, if it be correct to say that odours set up ultra-violet rays in the ether, we must be equally ready to credit them with setting up infra-red rays also! But there is another, and perhaps a stronger, objection to the ultra-violet theory. In the interesting and highly instructive schema drawn up by Heyninx of the wave-lengths of ultra-violet absorbed by odours, we find one or two discrepancies of a serious character. For example, iodoform and cinnamic aldehyde show absorption-bands occupying nearly the same position on the spectrum; and presumably, therefore, these substances have the same molecular vibration-rate. Yet their odours are not at all alike! Again, acetone-methylnonic and butyric acids have _precisely_ the same absorption bands, and yet they also exhale totally different odours. But the most serious discrepancy remains. The absorption bands of hydrocyanic acid and watery vapour (steam) have precisely the same position in the spectrum, yet one of these has a highly characteristic odour, and the other has none at all! It is rather difficult, in view of these findings, to believe that this absorption phenomenon can have anything to do with the quality of odour. My friend Mr. T. H. Fairbrother writes regarding this controversy:—