Measurement of rotatory dispersive power in the visible and ultra-violet regions of the spectrum

The following is a brief preliminary account of improvements effected in the method of determining rotatory dispersive power which have made it possible to observe accurately not only in the bright regions of the visible spectrum, but throughout the scale from the region of the lithium red line into that commanded by the photographic plate. Two methods have generally been used for the purpose, namely, (1) Broch’s method, in which a spectroscope is arranged in series with the polarimeter and a narrow strip of a continuous spectrum is picked out for observation—a method which is much improved by using a constant-deviation spectroscope in place of one of the variable-deviation type, and .(2) Landolt’s method, in which a white light is reduced by means of filters to approximate homogeneity in the red, green, light-blue, or dark-blue parts of the spectrum. Neither method fulfils the fundamental condition that the field of the polarimeter shall be uniformly lighted with monochromatic light—many of the measurements that have been made, therefore, possess only a qualitative value. A much better method is due to the late Sir William Perkin, who introduced the use of a spectroscope-eyepiece as a means of purifying the sodium light, and used it on a limited scale for measuring rotatory dispersive power in the red (lithium), yellow (sodium), and green, thallium) parts of the spectrum.