T HE investigation of the sensory capacities of various animal forms has long been a problem of interest to the biologist, general zoologist, and student of comparative psychology. Answers to questions of whether different animals see form and color, are able to distinguish various odors, or are sensitive to musical sounds, have often been sought by many investigators. Not only has the interest been in determining whether or not general sensory fields are functional in particular organisms, but also in finding out within what limits of variation of the external energy the animal may show awareness. Again, problems of how fine are the discriminations that can be made between different qualities of the stimulation energies have been assayed. A great deal of data has been amassed with regard to the sensory capacity of all sorts of organisms. In considering the action of various forms of energy on the sense organs, two types of problems may be investigated. First, the sensory limits, in terms of the range of variation of the stimulus energy, may be determined, as well as the sensory acuity, or the ability to perceive differences between quantitative manifestations of the same type of energy. Second, neurological problems, concerned with a description of the behavior of the sensory neurones when acted upon by an external stimulus, may be studied. A general way of attacking the former problem, that of measuring sensory limens and acuity, is by the discrimination method. This consists in presenting the animal with successive situations, alike, as nearly as possible, in all respects, except in the quantity or quality of some energy manifestation peculiar to the sensory capacity under investigation. A measure of the animal's perception of the energy is taken by the efficiency with which it comes to associate a certain response, (usually locomotor), with certain relationships between the stimulus energy manifestations. For example, in investigating the question of color vision in chicks, each animal might be trained to go to one side of the reaction box on being presented with the red light, and to go to the other side when the green light appears. Training of the animal usually takes a number of trials which are spread out over a considerable period of time. Before the results of any discrimination method investigations can be accepted, three general types of controls must have been made during the course of the experiment. These include, i. The organism must be kept as nearly a constant factor as the progress of maturation and growth will allow during the course of the investigation. m. The apparatus for presenting the stimulus energies in similar situations must be simple, present no distractions or secondary cues, and afford means of building up a clearly differentiated response for each different manifestation of energy.