spot on a scale. A curve of variation of the indices with the different strengths of solution may thus be easily obtained.
3. Effect of temperature
The index of a liquid is increased and the critical angle decreased with the lowering of temperature. Hot water is poured into the cylinder, and the cylinder rotated till the angle of incidence is just a little less than the critical angle for the particular temperature. The liquid is stirred at intervals, and a thermometer bulb placed in the same horizontal layer of the liquid through which light is passing. As the liquid slowly cools down, the critical angle is decreased, and at a certain temperature the image suddenly disappears. The corresponding temperature of the liquid is now observed. The angle is then decreased by a small known amount, and the new temperature for total reflection again observed. In this way the slight variation of the angle with the variation of temperature is found. The absolute value of the critical angle for a standard temperature is known from a previous experiment. Hence the indices for different temperatures may be easily deduced.
4. On the determination for different rays and the dispersive power
The value for the D line having been accurately determined by the method of repetition, the values for the other rays are found in the following way:—
Sunlight may be used for the experiment; the image falls on the slit of a direct vision spectroscope. A spectrum is thus formed, in which the well-