The factors found by the Hotelling method of principal components present the same necessity for rotation as those found by the Thurstone method of multiple factors.

The Thurstone method of multiple factors is here found superior to the Hotelling method of principal components, and is suggested as the method of choice for all problems requiring the determination of elemental components from a group of related variables, since it was found on every point of comparison that, when applied to a correlation matrix of unknown communalities, the Thurstone method was more accurate than the Hotelling method, and also required much less time.

When the first of a uniform chain of neurones is excited with a stimulus intensity S1, it is found that, according to the nature of the chain, either the excitation is always transmitted with decreasing intensity, or else the intensity for succeeding neurones of the chain approaches a fixed finite value, characteristic of the chain and independent of S1, whenever S1 exceeds a certain “threshold” value.

A method is developed for extending any type of factor solution to new tests. The theoretical basis for this approximating scheme is thoroughly investigated, and then a simplification in the technique is introduced for practical purposes. An example is presented which illustrates the procedure of extending a factor solution to three new tests simultaneously.

A battery of thirteen tests of motor abilities are analyzed by means of the Thurstone factor methods. Interpretations of the nature of the six factors extracted are offered, with reservations. The high degree of specificity of motor abilities results in very narrow group factors.

A formula for the stimulus is derived. The essential postulates and assumptions are based on Gestalt theory.

The properties of a simple neural mechanism are developed theoretically and interpreted in terms of observable variables. The results are discussed in connection with experimental data in cases for which the latter are available.