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VOLTAIC ELECTRICITY. XX. 



GRAHAM BELL'S PHOTOPHONE MOLECULAU MUSIC. 



HAVING constructed an efficient cell, as described in our pre- 

 vious lesson, Professor Bell's next task was to devise apparatus 

 by means of which the intensity of a beam of light can be con- 

 trolled by the voice (see Fig. 93). The first forms contrived con- 

 sisted of a vibrating diaphragm, D, with month-piece, c, attached. 

 At the back of this diaphragm were placed two gratings, the 

 ne fixed and immovable, B, t'ho other, A, attached to the centre 



which the speculum of a reflecting telescope is silvered. This 

 flexible mirror is furnished at its back with a speaking tube, o, 

 so that every utterance of the speaker causes the mirror to 

 vibrate. Such vibrations will, of course, cause the surface to 

 become alternately concave and convex, so that a ray of light 

 reflected by the mirror is alternately condensed and spread out. 

 The intensity of the light falling upon the selenium receiver is 

 thus caused to vary with the variations of the speaker's voice. 

 It is found in practice that the great heat of a ray of sunlight 

 concentrated by a lens on such a thin diaphragm is very apt to 

 fracture it, so that a cell, r. containing a concentrated solution 



