highway for investigation is without doubt Professor Zwaardemaker, of Utrecht. The invention of his first olfactometer in 1888 and the appearance in 1895 of his great work _Die Physiologie des Geruchs_ have served to give the physiology of the sense of smell an assured status and to open the way anew for much fruitful investigation, while a number of inquirers in many countries have had their attention directed to the elucidation of this sense. 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