IT is well known that the photometry of light sources which are different in color is subject to great errors. In order to obviate this difficulty some form of radiometer which, unlike the eye, functions independently of the frequency (wave-length) and the intensity of the stimulus, has been advocated.
A bolometer or a thermopile is the simplest radiometer now available. In order to make it conform with the behavior of the average normal eye when subjected to a stimulus which doe's not deviate much from a certain specified intensity, it is necessary to interpose between the radiometer and the source of light (which is to be tested) an absorption screen which is opaque to all the infra-red and ultraviolet radiations and which transmits the visible radiations in proportion to the luminosity curve of the average normal eye. In other words., the spectral transmission curve of the absorbing medium must be an exact copy of the luminosity curve of the average normal eye, when subjected to a stimulus of a given intensity.
All this sounds very enticing in theory, but in practice the radiometer has various limitations which, as will be shown presently, may restrict its use to special problems which can be investigated at leisure, when conditions are favorable for operating the radiometric outfit.
This paper makes no pretence in giving the details of the development of the aforementioned ideas. In the earliest work by F6ry 1 an absorption cell containing a solution of copper acetate was used, and no attempt was made to have its transmission curve coincide with the luminosity curve.
Recently Karrer 2 has made a series of radiant luminous efficiency measurements on various light sources by determining the ratio of energy * Communicated by the Author.