IN a general way it is well known that exceedingly weak solutions of many substances are sufficient to provoke sensations of taste. It is also known that weak tastes which some individuals are able to detect or to recognize correctly go quite unobserved by others. The same thing is true of differences between tastes. The connoisseur is sensitive to minute differences in the flavor of wine, tobaccos, and sauces. Through practice the expert taster of these substances acquires a skill which is quite incomprehensible to the inexperienced. In part only is such skill a matter of special sensory activity. It is in large measure a matter of perception rather than one of sensation,—a knowledge of what signs to look for and how to interpret these signs,—rather than an increased sensitiveness to stimuli. In the same way the skilled gardener, hunter, or scout is alert to the significance of particular signs and clues and this alertness and apt interpretation may make him appear to have senses of exceeding acuteness, although this may by no means be borne out by actual measurements. The psychological problems involved in the measurement of keenness of taste are mainly two in number. One problem concerns itself with the question, What is the faintest stimulus that can be sensed,—the weakest taste that can be appreciated? The other concerns itself with the sensitivity to difference between tastes, and would be expressed by some such question as, How slight a change in the amount or intensity of the stimulus is required for one to be able to perceive a change in the intensity of the taste sensation? Unfortunately for our knowledge of tastes, both these problems are very difficult to approach experimentally. Whether or not a given weak stimulus will provoke a taste sensation depends on very many things other than the strength of the solution. The amount of solution applied, the extent of surface excited, the duration of the application, the temperature of the solution, the state of rest or movement of the sense organ, and the nature of preceding stimuli, among other things, are important. _The Threshold of Taste Sensation_ Numerous investigations have concerned themselves with the task of discovering the weakest solution of various substances that will provoke their respective sensory qualities. The main results of these studies have been the demonstration of two facts, namely, that exceedingly weak stimuli may arouse sensations of taste and that this minimum solution varies greatly in amount according to the substance which is in question. Valentin, in 1842, measured the lower threshold for solutions of sugar, salt, quinine, and sulphuric acid and found the following proportions to represent the least amounts able to arouse the corresponding sensation: Sugar 1.200 parts to 100 parts of water Salt 0.300 Acid 0.001 Quinine 0.003