[Illustration: FIG. 76.--SIDE VIEW OF THE OPTOPHONE] There is an electric lamp in the machine before which there is a disk that is revolved by a small motor. In this disk there are five circular rows of slots and the light shining through these slots is cut up into five pulsating beams of light. These beams are brought to a focus in a vertical row upon the type page. From the paper they are reflected to a selenium cell or bridge. The selenium bridge forms part of the circuit of a telephone receiver and the diaphragm is thus made to vibrate at the same frequency as the light beams do. By varying the number of slots in each row in the disk the beams of light are given different periods of pulsation or vibration and they produce a sound chord or “scala” in the telephone receiver. The speed of the disk and the disposition of the slots is so chosen that the notes produced are _G_ _C′_ _D′_ _E′_ _G′_ of the musical scale. Only white paper reflects the light beams; the black surface of the printed type absorbs them. Thus, as the row of beams is swept across a line of printed matter, the beams will be extinguished in various orders of succession, or simultaneously in accordance with the shape of the particular type-character they encounter. The middle three beams correspond to notes _C′_, _D′_ and _E′_, and play upon the small letters, while _G′_ plays up the upper part of capital letters, and _G_ upon the tails of such letters as _y_, _p_, etc. If the scala passes over the letter “V,” for instance, first the top note _G′_ is silenced, then _E′_, _D′_, _C′_, _D′_, _E′_ and _G′_ in succession. This arrangement constitutes what is known as the “white sounding” optophone, because the full chord is sounded constantly, except when the type matter is encountered. To simplify the reading an improved type of optophone has been made, which is known as the “black-reading” optophone, With this machine there is no sound produced except when the type is encountered. The letter “V” is then identified by the _sounding_, instead of the _silencing_ of the notes _G′_, _E′_, _D′_, _C′_, _E′_ and _G′_. The letter “A” produces the sounds _C′_, _D′_, _DE′_, _DG′_, _DE′_, _D′_ and _C′_. This result is obtained by using two selenium bridges, as shown better in the side view, Figure 76. There is a concave reflecting lens, which reflects half of the light upon the second cell, known as the balancer selenium bridge. Electric current passing through the balancer opposes the current passing through the main selenium bridge, and hence there is silence in the telephone receiver when the scala passes over plain white paper, but when type is encountered and certain of the beams are not reflected against the main selenium bridge the sounds are produced through the balancer bridge.