Notwithstanding, however, the amount of work which has been done in this field during recent years, it cannot be said that the body of assured conclusions so far reached is large.  The most fundamental principles of olfactory physiology and psychology are still somewhat vague and uncertain.  Although sensations of smell are numerous and varied, in this respect approaching the sensations of vision and hearing, smell still remains close to touch in the vagueness of its messages (while the most sensitive of the senses, remarks Passy, it is the least precise), the difficulty of classifying them, the impossibility of so controlling them as to found upon them any art.  It seems better, therefore, not to attempt to force the present study of a special aspect of olfaction into any general scheme which may possibly not be really valid.

The earliest and most general tendency in regard to the theory of smell was to regard it as a kind of chemical sense directly stimulated by minute particles of solid substance.  A vibratory theory of smell, however, making it somewhat analogous to hearing, easily presents itself.  When I first began the study of physiology in 1881, a speculation of this kind presented itself to my mind.  Long before Philipp von Walther, a professor at Landshut, had put forward a dynamic theory of olfaction (Physiologie des Menschen, 1807-8, vol. ii, p. 278).  “It is a purely dynamic operation of the odorous substance in the olfactory organ,” he stated.  Odor is conveyed by the air, he believed, in the same way as heat.  It must be added that his reasons for this theory will not always bear examination.  More recently a similar theory has been seriously put forward in various quarters.  Sir William Ramsay tentatively suggested such a theory (Nature, vol. xxv, p. 187) in analogy with light and sound.  Haycraft (Proceedings of the Royal Society of Edinburgh, 1883-87, and Brain, 1887-88), largely starting from Mendelieff’s law of periodicity, similarly sought to bring smell into line with the higher senses, arguing that molecules with the same vibration have the same smell.  Rutherford (Nature, August 11, 1892, p. 343), attaching importance to the evidence brought forward by von Brunn showing that the olfactory cells terminate in very delicate short hairs, also stated his belief that the different qualities of smell result from differences in the frequency and form of the vibrations initiated by the action of the chemical molecules on these olfactory cells, though he admitted that such a conception involved a very subtle conception of molecular vibration.  Vaschide and Van Melle (Paris Academy of Sciences, December 26, 1899) have, again, argued that smell is produced by rays of short wave-lengths, analogous to light-rays, Roentgen rays, etc.  Chemical action is however, a very important factor in the production of odors; this has been well shown by Ayrton (Nature, September 8, 1898).  We seem to be forced in the