nt of this point is communicated to the aluminium
arm D, which is pivoted at C. As will be seen the telephone receiver is of
special construction, it containing only one coil and therefore only one
core; by this means the movement of the diaphragm is centralised. The coil
is wound for a resistance of about 200 ohms, and the diaphragm should be
fairly thin but very resillient.

[Illustration: FIG. 27.]

To the free end of D is fastened the mirror T, made from thin diaphragm
glass about 1-1/2 centimetres diameter, and having a focal length of 40
inches. Light from the lamp L is transmitted by the lens N in a parallel
beam to the mirror which {52} concentrates it to a point upon a hole
1/100th of an inch in diameter in the screen J. As the telephone diaphragm
vibrates under the influence of the received signals the arm, and
consequently the mirror, vibrates also, and the hole in the screen J is
constantly being covered and uncovered by the spot of light. It will be
seen from Fig. 27 that the ratio between the centre of the mirror and the
pivot C, and C and the steel point P is 10:1, so that if a movement of
1/20000th of an inch is obtained at the centre of the diaphragm the mirror
will move 1/2000th of an inch; and as the focal length of the mirror is 40
inches a movement of 1/50th inch is given to the spot of light.

This receiver is capable of working at a fairly high speed, as the inertia
of the moving parts is practically negligible; the weight of the arm and
mirror being less than 20 grains. The hole in the screen is made slightly
less in diameter than the traverse of the revolving cylinder, the slight
distance between the cylinder and the screen allowing the light to disperse
sufficiently to produce a line on the film of about the right thickness.

There are two other possible means of photographically receiving the
picture that upon investigation may yield some results; but it is doubtful
whether the current available, even that obtained from a telephone relay,
will be sufficient to produce the desired magnetic effect, and the {53}
insertion of a second relay would detract greatly from the efficiency by
decreasing the speed of working. If rays of monochromatic light from a lamp
L, Fig. 28, pass through a Nicol prism P (polarising prism), then through a
tube containing CS_2 (carbon bisulphide), afterwards passing through the
second prism P' (analysing prism), and if the two Nicol prisms are set at
the polarising an