132 
SCIENCE. 
a residue which proved upon examination to be what he had been 
in search of— a new elementary substance. The chemical proper- 
ties of this new element were found to resemble those of tellurium 
in so remarkable a degree that Berzelius gave to the substance the 
name of "Selenium," from the Greek word selene, the moon — 
(“ tellurium," as is well known being derived from tellus, the earth, 
Although tellurium and selenium are alike in many respects, they 
differ in their electrical properties ; tellurium being a good conduc- 
tor of electricity, and selenium, as Berzelius showed, a non-con- 
ductor. Knox discovered, in 1837, that selenium became a conduc- 
tor when fused ; and Hittorff, in 1852, showed that it conducted, at 
ordinary temperatures, when in one of its allotropic forms. When 
selenium is rapidly cooled from a fused condition, it is a non-con- 
ductor. In this, its vitreous form, it is of a dark-brown color, 
almost black by reflected light, having an exceedingly brilliant 
surface. In thin films it is transparent, and appears of a beautiful 
ruby red by transmitted light. When selenium is cooled frcm a 
fused condition with extreme slowness, it presents an entirely dif- 
ferent appearance, being a dull lead color, and having throughout 
a granulated or crystalline structure, and looking like a metal. In 
this form it is perfectly opaque to light, even in very thin films. 
This variety of selenium has long been known as “ granular" or 
" crystalline " selenium, or, as Regnault called it, " metallic " sele- 
nium. It was selenium of this kind that Hittorff found to bfe a 
conductor of electricity at ordinary temperatures. He also found 
that its resistance to the passage of an electrical current diminished 
continuously by heating up to the point of fusion, and that the 
resistance suddenly increased in passing from the solid to the 
liquid condition. It was early discovered that exposure to sunlight 
hastens the change of selenium from one allotropic form to an- 
other; and this observation is significant in the light of recent dis- 
coveries. 
Although selenium has been known for the last sixty years it has 
not yet been utilized to any extent in the arts, and it is still consid- 
ered simply as a chemical curiosity. It is usually supplied in the 
form of cylindrical bars. These bars are sometimes found to be in 
the metallic condition ; but more usually they are in the vitreous or 
non-conducting form. It occurred to Willoughby Smtih that, on 
account of the high resistance of crystalline selenium, it might be 
usetully employed at the shore-end of a submarine cable, in his 
system of testing and signalling during the process of submersion. 
Upon experiment, the selenium was found to have all the resist- 
ance required^-some of the bars employed measuring as much as 
1400 megohms — a resistance equivalent to that which would be of- 
fered by a telegraph wire long enough to reach from the earth to 
the sun ! But the resistance was found to be extremely variable. 
Experiments were made to ascertain the cause of this variability. 
Mr. May, Mr. Willoughby Smith’s assistant, discovered that the 
resistance was less when the selenium was exposed to light than 
when it was in the dark. 
In order to be certain that temperature had nothing to do with 
the effect, selenium was placed in a vessel of water, so that the 
light had to pass through from one to two inches of water in order 
to reach the selenium. The approach of a lighted candle was 
found to be sufficient to cause a marked deflection of the needle of 
the galvanometer connected with the selenium, and the lighting of 
a piece of magnesium wire caused the selenium to measure less 
than half the resistance it did the moment before. 
These results were naturally at first received by scientific men 
with some incredulity, but they were verified by Sale, Draper, Moss 
and others. When selenium is exposed to the action of the solar 
spectrum, the maximum effect is produced, according to Sale, just 
outside the red end of the spectrum, in a point nearly co-incident 
with the maximum of the heat ra>s; but, according to Adams, the 
maximum effect is produced in the greenish-yellow or most lumin- 
ous part of the spectrum. Lord Rosse exposed selenium to the 
action of non-luminous radiations from hot bodies, but could pro- 
duce no effect ; whereas a thermopile under similar circumstances 
gave abundant indications of a current. He also cut off the heat- 
rays from luminous bodies by the interposition of liquid solutions, 
such as alum, between the selenium and the source of light, with- 
out affecting the power of the light to reduce the resistance of the 
selenium ; whereas the interposition of these same substances al- 
most completely neutralize the effect upon the thermopile. Adams 
found that selenium was sensitive to the coldlight of the moon, and 
Werner Siemens discovered that, in certain extremely sensitive 
varieties of selenium, heat and light produced opposite effects. In 
Siemens's experiments, special arrangements were made for the 
purpose of reducing the resistance of the selenium employed. Two 
fine platinum wires were coiled together in the shape of a double 
flat spiral in the zig-zag shape, and were laid upon a plate of mica 
so that the discs did not touch one another. A drop of melted 
selenium was then placed upon the platinum-wire arrangement, 
and a second sheet of mica was pressed upon the selenium, so as 
to cause it to spread out and fill the spaces between the wires. 
Each cell was about the size of a silver dime. The selenium cells 
were then placed in a paraffine bath, and exposed for some hours 
to a temperature of 210° C., after which they were allowed to cool 
with extreme slowness. The results obtained with these cells were 
very extraordinary ; in some cases the resistance of the cells, when 
exposed to light, was only one-fifieenth of their resistance in the 
dark. 
Without dwelling farther upon the researches of others, I may say 
that the chief information concerning the effect of light upon the 
conductivity of selenium will be found under the names of Wil- 
loughby Smith, Lieutenant Sale, Draper and Moss, Professor W. 
G. Adams, Lord Rosse, Day, Sabini, Dr. Werner Siemens and 
Dr. C. W. Siemens. All observations by these various authors 
had been made by means of galvanometers ; but it occurred to me 
that the telephone, from its extreme sensitiveness to electrical in- 
fluences, might be substituted with advantage. Upon considera- 
tion of the subject, however, I saw that the experiments could not 
be conducted in the ordinary way for the following reason : The 
law of audibility of the telephone is precisely analogous to the law 
of electric induction. No effect is produced during the passage of 
a continuous and steady current. It is only at the moment of change 
from a stronger to a weaker state, or vice versa, that any audible 
effect is produced, and the amount of effect is exactly proportional 
to the amount of variation in the current. It was, therefore, evi- 
dent that the telephone could only respond to the effect produced 
in selenium at the moment of change from light to darkness, or 
vice versa ; and that it would be advisable to intermit the light 
with great rapidity, so as to produce a succession of changes in 
the conductivity of the selenium, corresponding in frequency to 
musical vibrations within the limits of the sense of hearing. For 
I had often noticed that currents of electricity, so feeble as to pro- 
duce scarcely any audible effects from a telephone when the circuit 
was simply opened or closed, caused very perceptible musical 
sounds when the circuit was rapidly interrupted, and that the 
higher the pitch of sound the more audible was the effect. I was 
much struck by the idea of producing sound by the action of light 
in this way. Upon farther consideration it appeared to me that 
all the audible effects obtained from varieties of electricity could 
also be produc d by variations of light acting upon selenium. I 
saw that the effect could be produced at the extreme distance at 
which selenium would respond to the action of a luminous body, 
but that this distance could be indefinitely increased by the use of 
a parallel beam of light, so that we could telephone from one place 
to another without the necessity of a conducting wire between the 
transmitter and receiver. It was evidently necessary, in order to 
reduce this idea to practice, to devise an apparatus to be operated 
by the voice of a speaker, by which variations could be produced 
in a parallel beam of light, corresponding to the variations in the 
air produced by the voice. 
I proposed to pass light through a large number of small orifices, 
which might be of any convenient shape, but were preferably in 
the form of slits. Two similarly perforated plates were to be em- 
ployed. One was to be fixed and the other attached to the centre 
of a diaphragm actuated by the voice, so that the vibration of the 
diaphragm would cause the movable plate to slide to and fro over 
the surface of the fixed plate, thus alternately enlarging and con- 
tracting the free orifices for the passage of light. In this way the 
voice of a speaker could control the amount ot light passed through 
the perforated plates without completely obstructing its passage. 
This apparatus was to be placed in the path of a parallel beam ot 
light, and the undulatory beam emerging from the apparatus 
could be received at some distant place upon a lens, or other ap- 
paratus, by means of which it could be condensed upon a sensi- 
tive piece of selenium placed in a local circuit with a telephone and 
galvanic battery. The variations in the light produced by the 
voice of the speaker should cause corresponding variations in the 
electrical resistance of the selenium employed : and the telephone 
in circuit with it should reproduce audibly the tones and articula- 
tions ot the speaker's voice. I obtained some selenium for the 
purpose of producing the apparatus shown ; but found that its re- 
sistance was almost infinitely greater than that of any telephone 
that had been constructed, and I was unable to obtain any audible 
effects by the action of light. I believed, however, that the ob- 
stacle could be overcome by devising mechanical arrangements 
for reducing the resistance of the selenium, and by constructing 
special telephones for the purpose. I felt so much confidence in 
this that, in a lecture delivered before the Royal Institute of Great 
Britain, upon the 17th of May, 1878, I announced the possibility 
of hearing a shadow by interrupting the action of light upon sel- 
enium. A few days afterwards my ideas upon this subject re- 
ceived a fresh impetus by the announcement made by Mr. Will- 
oughby Smith betore the Society of Telegraph Engineers that he 
had heard the action ot a ray of light falling upon a bar of crys- 
talline selenium, by listening to a telephone in circuit with it. 
It is not unlikely that the publicity given to the speaking tele- 
phone during the last few years may have suggested to many 
minds in different parts of the world somewhat similiar ideas to my 
own. 
Although the idea of producing and reproducing sound by the 
action of light, as described above, was an entirely original and 
independent conception of my own, I recognize the fact that the 
knowledge necessary for its conception has been disseminated 
throughout the civilized world, and that the idea may therefore 
have occurred to many other minds. The fundamental idea, on 
which rests the possibility of producing' speech by the action of light, 
is the conception of what may be termed an undulatory beam of 
light in contradistinction to a merely intermittent one. By an undu- 
latory beam of light, I mean a beam that shines continuously upon 
the selenium receiver, but the intensity of which upon that receiver 
is subject to rapid changes, corresponding to the changes in the 
vibratory movement of a particle of air during the transmission o 
