156 T. G. Mendenhall on the iSensorium. the hands of Huggins, Secchi, Young, and others, the spectro¬ scope, that marvel of modern science, has yielded satisfactory testimony not only in regard to such stars as are reached by our unassisted vision, but even respecting the telescopic nebulae, apparently on the outskirts of the visible creation. A detailed account of these wonderful achievements would not comport with our present purpose. Such results, however, as bear di¬ rectly upon the theory of Laplace will be briefly noted. 1. The ring nebula in Lyra, the Dumb-bell nebula, the great nebula in Orion, and others which might be named, are not, as was but recently believed, extremely remote sidereal clusters ; but their light undoubtedly emanates from matter in a gaseous form. 2. “ According to Lord Rosse and Professor Bond the brighter parts near the trapezium [in the nebula of Orion] consist of clustering stars. If this be the true appearance of the nebula under great telescopic power, then these discrete points of light must indicate separate and probably denser portions of the gas, and that the whole nebula is to be regarded rather as a system of gaseous bodies than as an unbroken vaporous mass.”* 3. Progressive changes in the physical condition of certain nebula are clearly indicated by the fact that nuclei have been established which, as shown by their spectra, are not wholly gaseous, but have passed, at least partially, to the solid or liquid form. 4. The spectroscopic analysis of the light of several comets reveals a constitution similar to that of the gaseous nebulæ. The spectroscope, then, has demonstrated the present existence of immense nebulous masses, such as that from which Laplace supposed the solar system to have been derived. It has shown, moreover, a progressive change in their physical structure, in accordance with the views of the same astronomer. In short, the evidence afforded by spectrum analysis in favor of the neb¬ ular hypothesis is cumulative, and of itself sufficient to give this celebrated theory a high degree of probability. Art. XXII.—Experiments on the time required to communicate impressions to the Sensarium, and the reverse; by T. C. Men¬ denhall, Columbus, Ohio. I propose in this paper to give a few of the results of some experiments, carried on during the last fall and winter, having in view the determination of the time occupied in the perception of an object and the response to that perception, by an action performed in the simplest possible manner ; also, to estimate the * Monthly Notices of the R. A. S., vol. xxv, p. 166.