Page:On the Influence of the Thickness of Air-space on Total Reflection of Electric Radiation.djvu/5

304

This is the case only when the two prisms are made of the same substance. If the second prism be made of sulphur, or of any other substance which has either a lower or a higher refractive index, there is always found a reflected portion even when the two prisms are in contact.

Another interesting observation can be made by separating the prisms for total reflection. There would now be no transmitted portion. But if a thin piece of cardboard or any other refracting substance be now interposed in the air-space, a portion of the radiation will be found to be transmitted, and it will be found necessary to separate the prisms further to reduce the transmitted portion to zero.

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 vary 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 The cards were then carefully withdrawn, leaving the prisms separated by a thickness of air equal to the thickness of the given number of cards.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.