The delicacy of discrimination of tones is greatest in the mid-region of hearing; and there is much individual variation in accuracy of ear. I have made experiments on many individuals to test their powers in this respect. I found some who were unable, in the mid-region of hearing, to state which was the higher of two notes sounded on a violin, the tones of which were separated by a major third, and in one case by a fifth. With notes on the piano the discrimination was more delicate, and yet more delicate when the notes were sung. In such cases tone-discrimination is deficient; and between these and the musician, who is stated to be able to distinguish tones separated by only 1/64 of a tone, there are many intermediate stages. It is beyond my purpose to describe, in more than a very general way, the nature of the auditory apparatus of man. The vibrations of the air are received by the drum-membrane, which lies in the auditory passage. From this it is transmitted, by a chain of small bones, to the inner auditory apparatus. This consists of two small membranous sacs, with one of which three membranous looped tubes, the semicircular canals, are connected; with the other is connected a spiral tube, the cochlear canal. These membranous sacs and canals are filled with fluid, and are surrounded by the fluid which fills the bony cavity in which they lie. This bony cavity has two little windows, one oval and the other round, across each of which a membrane is stretched. The oval membrane is in connection with the chain of auditory bones; and when this is made to vibrate in and out, the membrane of the round window vibrates out and in. Thus the fluid around and within the membranous sacs and canals is set in vibration. And the parts are so arranged that the vibrations, in passing from the oval to the round membrane, must run up one side and down the other side of the cochlear canal. As they run down they set in vibration a delicate membrane which is supported on beautiful arched rods (the organs of Corti). And this membrane contains a number of special hair-cells, so called because they bear minute hair-like structures. These are the special end-organs of hearing. It has been suggested that the fibres of the membrane on the arched rods, which are of different lengths and may be stretched with differing degrees of tension, respond to vibrations of different pitch. Thus the hair-cells on that particular part of the membrane would be stimulated, and the note might be appreciated in its true position in the scale.