_Refraction and Dispersion._--The quantitative properties of glass, governing its effect upon incident and transmitted light, are, of course, of fundamental importance in all its optical uses. The fundamental optical constant of each variety of optical glass is known as its refractive index; this number really represents the ratio of the velocity with which light waves are propagated through the glass to the velocity with which they travel through free space. Not only does this ratio vary with every change in the chemical composition and physical condition of the glass, but it also varies according to the length of the light waves themselves. In other words, the short waves of blue light are transmitted through glass with a different velocity from that with which the longer waves of red light are transmitted. The consequence is that when a beam of white light is passed through a prism it is split up and spread out into a number of beams representing all the colours of the spectrum in their proper order, the blue light suffering the greatest deflection from its original path, while the red light suffers least deflection. Both the actual and relative amount by which light rays of various colours are deflected under such circumstances depends upon the nature of the glass in question; therefore, to fully characterise the optical properties of a given kind of glass it is necessary to state not only its refractive index but to specify the refractive indices for a sufficient number of different wave-lengths of light, suitably distributed through the spectrum. For this purpose a number of well-marked spectrum lines have been chosen, the systematic use of the particular set of lines which is now usually employed being due to the initiative of Abbé and Schott at Jena, who initiated the system of specifying the optical properties of glass in this way. The actual lines chosen are the line known as A′, corresponding to a wave-length of 0·7677 micro-millimetres, and the lines known as C, D, F, and G′, whose wave-lengths, in the same units, are 0·6563, 0·5893, 0·4862, and 0·4341 respectively. The A′ line, however, lies so near the extreme red end of the spectrum that the data concerning it are seldom required.