Determination of the Indices of Refraction.—For the determination of the index the prism-method is not very suitable. I found the following to yield good results, the method depending on the determination of the critical angle. Two semi-cylinders of the given substance separated by an air-space are
Fig. 5.
placed on the platform. When the radiator is placed at the principal focus of one of the semi-cylinders the rays emerge parallel into the air-film, and are then focussed on the receiver by the second semi-cylinder. A metallic plate with a narrow rectangular opening is interposed between the semi-cylinders to serve as a diaphragm, and cut off all but the central rays. As the platform is rotated, the incident angle on the plane surface separating the two media is gradually increased till the rays undergo total reflexion. When this is the case the receiver, which is placed opposite the radiator, suddenly ceases to respond. The trouble of following the deviated ray is thus
Fig. 6.
(The dotted lines show the two positions of the air-film for total reflexion.)
obviated; the reading is also well defined as the transition from refraction to total reflexion is abrupt. The index-reading is now taken, and the cylinders rotated in an opposite direction till total reflexion takes place a second time. The difference of readings as given by the index in the two positions is evidently equal to twice the critical angle. Hence the value of the index can easily be deduced.
A preliminary experiment gives the approximate value of the index, from which the focal distance of the semi-cylinder is roughly calculated. The spark-gap of the radiator is placed
