A photometer is an apparatus which can compare the intensities of two sources of light. In this specimen, devised and made by the opticist Dr. J.G. Hofmann from Paris, the instrument is placed between the two light sources to be compared. The light intensities are adjusted until equal brightness is perceived by using an adjustable (purple) filter. The reading of the filter is a measure for the relative intensity. Because of this relative measure this type was rather unsuccessful.
Photometer
circa 1870