means nothing of itself, but if it be broken up into long and short hums, according to the Morse code of signals, it will be heard by the receiving clerk as a message, which can be written down as he listens to it. This breaking up is done by manipulating the key M1 which is like the ordinary Morse signalling key, but acts by inserting a screen o, in the track of the light before it reaches the occulting wheel, thus cutting off the light altogether, and stopping the note. When the key is down the screen is interposed, and there is a pause in the hum heard at the receiving station. When it is up, the hum is heard, and the length of these broken sounds indicates whether the signal is a "dot" or "dash." When two or more transmitters S1 S2 are working together, so as to send several messages over the same wire simultaneously, the revolving wheels are arranged to occult at different rates, so as to make the period of the flashes different for each transmitter. The receiving telephones T2 Ti are then adjusted to respond only to their own particular note, and thus the different messages are sifted from one another.

We may also add that M. Paul Samuel has arranged that the mirror galvanometer used in signalling through long submarine cables shall record its message on paper. This is done by causing the right and left movements of the beam of light constituting the ordinary message to fall on selenium cells each in circuit with a battery and an ink recording instrument. The fall of the light on the right and left cells causes the right and left recorders to mark the signal. The inspired ray of the photophone naturally leads to that great electrical invention of the future, the trans

mission of light by wire. One step towards the solution of the problem how to send a picture of an image of an object to a distant place by means of electricity, has been made by Mr. Shellford Bidwell. This apparatus is a combination of the Bakewell copying telegraph and the selenium cell, which Professor Graham Bell has so ingeniously applied to the transmission of speech by telephone along a beam of light, and reference to Fig. 46, will help to explain the

process. In this figure P is a marking stylus of platinum, with its point resting on a sheet of paper laid on a metal tablet м. The stylus is connected

through a set of electric resistance coils to the positive pole of a voltaic battery в; and the tablet is connected through a galvanometer c to the negative pole of the same battery. There is thus a complete circuit for the current which flows from the stylus through the paper to the tablet and galvanometer. Now the paper is soaked in a solution of iodide of potassium similar to

that used in the old chemical telegraph of Bain, and the result is, that the current of electricity in passing through the paper decomposes the solution and liberates the iodine, which stains the paper brown. If, then, the stylus is drawn across the paper it will leave a brown trail behind it so long as the current passes. But should the current be stopped by any cause, this trail will cease and the paper will retain its original whiteness. It has been the aim of Mr. Bidwell therefore to make the luminous image which he wishes to transmit stop the current in such a way that the image itself will be delineated by the breaks in the marking of the stylus, and he does this by means of the second battery B' and the selenium cell s. This cell is composed of two fine spiral wires, wound very near together, and filled between with crystalline selenium. The wires are connected in circuit with the stylus P and the negative pole of the battery B'. Now it is evident that the current from the second battery tends to neutralize the current from the first in its decomposing action on the paper, and if strong enough will neutralize it altogether. Mr. Bidwell focusses a strong light or image on the cell s, then adjusts the resistance R, until the two currents exactly neutralize one another. Under these conditions the stylus will make no marks; but if the intensity of the light on the selenium diminishes, the resistance of the cell will increase, the current from B' will therefore be weakened, and the consequence will be that the current from B will predominate in the stylus and make a mark on the paper. The galvanometer serves to indicate the adjustment of the currents.

In the telephotograph this principle is carried out

according to Fig. 47 where p is the stylus as before, tracing a screw-line on a revolving barrel wrapped with prepared paper and connected to the marking battery B, through the galvanometer G. The transmitter consists of a barrel H, enclosing a selenium cells, connected to the battery B' and to

the stylus P. A fine pin-hole is drilled in the side of the barrel, which is revolved synchronously with the barrel of the receiver carrying the paper, and at every revolution the pin-hole crosses the surface of the cell in a spiral path. If now a luminous image of an object be focussed by the lens L on the barrel, just opposite the selenium cell, the pin-hole crossing the cell will let in a ray of light every time it crosses the image. In this way the pin-hole will trace out the shape of the image in parallel lines on the cell. But if the two batteries are so adjusted that when the ray of light falls on the cell the stylus will cease to mark, it

follows that the shape of the image will be delineated by the breaks in the parallel lines drawn on the paper by the stylus.

Fig. 48 represents a diamond pattern actually sent by the telephotograph of Mr. Bidwell. The luminous image focussed on the cell of the transmitter is seen on the left at A, and the picture of it outlined by breaks

in the chemical lines drawn on the receiver shown on

the right at B. With more delicate paper and a perfected apparatus Mr. Bidwell hopes to delineate light and shade in the image by fainter and deeper lines on the paper. The apparatus is one in its experimental state as yet, but the wires E w, Fig. 47, are intended to represent a telegraph line.