owing more itself to chemistry than to physics, in the latter especially has it been of assistance to the spectroscope, so that the experimenter is not dependent upon the observations of the moment to make his comparisons. The most considerable work of this kind has been done by Prof. Rowland within the last half-dozen years, in making remarkably large and detailed photographs of the solar spectrum, the spectrum itself, in its perfection and beauty, being due to the matchless gratings constructed under Rowland's directions. Photography has proved to be an unassailable recorder for all the natural sciences, and is likely to become more and more firmly established as such. Disputes over priority in discovery will become less frequent since investigations made in solitude will appeal to their photographic record as a safe witness, impartial and indisputable.
Another subordinate problem is to determine the intensity of sound in absolute measure. Acoustics has been studied with reference to the energy involved less than other branches of physics, although we easily recognize some transformations of such energy into mechanical in the phonograph and electrical in the telephone. But most determinations of the intensity of sound have been relative, by comparison of different sounds, or else the same sound at different distances or in different media. They have not been expressed in absolute units. Absolute values of radiant energy, in the form of heat and light, have been determined, but the methods have not been sufficiently simplified to make them readily applicable in experimenting. Temperatures are still given in arbitrary degrees, and intensity of illumination has no acceptable basis expressible in terms of the fundamental quantities mass, time, and distance, although several methods have been suggested in which the direct, subjective estimate of it by the eye plays no part.
This brings us to a consideration of the great service rendered to scientific investigation by an absolute system of units and measurements. Such systems were instituted by Gauss and Weber between the years 1834 and 1850, and their introduction was especially fruitful in the study of electricity. The mechanic was enabled by that means for the first time to compare the electric forces produced with the mechanical ones employed, and gained thereby for the first time a just estimate of the former. The adoption throughout the scientific world of the centimetre-gramme-second absolute system for all branches of science is by no means the least valuable outcome of the development which electrical science has undergone since 1850, for in the possibility of tracing back all natural phenomena to the three mechanical units of space, mass, and time, science received new evidence for the inherent unity and the mechanical character of all forces of
