Page:Collected Physical Papers.djvu/76

 VII

A SIMPLE AND ACCURATE METHOD OF DETERMINATION OF INDEX OF REFRACTION FOR LIGHT

The apparatus usually employed for the determination of the index of refraction is very elaborate and costly. Numerous adjustments have to be made, and a long time spent to secure accurate results. The following method for the determination of the optical index is a modification of the one which has for some time been employed by me in the determination of the index of electric refraction. The apparatus required is very simple, and I made a rough model of it at a trifling cost, and gave it to my pupils for trial. I was surprised to find how, even in their inexperienced hands, it gave results which would compare favourably with the various determinations of the indices hitherto made. The other advantages of this method are the quickness with which a determination can be made, and its adaptability to lecture demonstrations.

The method depends on the determination of the critical angle at which total reflection takes place. The principle of total reflection has also been employed by M. M. Terquem and Trannin, but the method described below is somewhat different, being much simpler, as no Collimator and observing Telescope are required. It has the additional advantage of being applicable to solids; and by a process of repetition, the value of the critical angle is obtained with a remarkably high degree of accuracy. (56)