Page:Collected Physical Papers.djvu/67

Rh Another interesting observation was made by separating the prisms just sufficiently for total reflection. There would now be no transmitted portion. But if a thin piece of cardboard or any other refracting substance were interposed in the air-space, a portion of the radiation was found to be transmitted; it was now necessary to separate the prisms further to reduce the transmitted portion to zero. This method opens out a possibility of determining the electric index of the interposed substance.

Having given a general account of the experiments I shall now describe the method of procedure. The radiator tube was provided with an ordinary lens whose focal distance for electric radiation is about 4 cm. The beam thus rendered approximately parallel fell perpendicularly on the face of the glass prism. The two prisms were made by cutting a cube of glass—an ordinary paper weight—across a diagonal. The size of the cube was 4·5 cm. on each side. One prism was fixed on the spectrometer circle; the other could be moved so as to change the thickness of the interposed air-space between the two sections very gradually. The separation was simply effected by means of ordinary cards. The cards used were of uniform thickness, each card being 0·45 mm. in thickness. A certain number of cards were taken and placed between the prisms with their surfaces in contact with the hypotenuses. The cards were then carefully withdrawn, leaving the prisms separated by a thickness of air equal to the thickness of the given number of cards. It Would, of course, be an improvement to have a micrometer screw by which the thickness may be gradually increased.