June 2, 1923] 
NATURE 
745 

area in the distance. The amount of light received by 
the selenium, placed, as it is, near the focus of a lens 
or mirror of definite aperture, diminishes, of course, as 
the distance is increased. Several factors determine 
the range of efficient transmission—the intrinsic 
brilliancy of the light source, the dimensions of the 
optical parts, the sensitivity of the selenium, and the 
number of stages of amplification which are used. No 
very conclusive tests of the maximum range of the 
photo-telephone have yet been carried out ; it may, 
nevertheless, be asserted with some confidence that, 
given sunlight and modern amplifying devices, it is 
probably the earth’s curvature which would impose a 
limit on the range of an instrument of quite reasonably 
small dimensions. 
It is of interest to compare the photo-telephone with 
the system of wireless telephony now so commonly 
used in broadcasting. In both, waves in the ether 
constitute the fundamental basis ; the medium is the 
same and the speed of propagation is the same. In 
both, speech vibrations modulate the intensity of the 
energy transmitted, and in both the results are made 
audible by changes of current in the receiving apparatus. 
The details are, of course, dissimilar. The radio- 
frequency waves are produced artificially, and are under 
control as regards wave-length ; the luminous waves 
are taken as we find them emitted from the source. 
The detectors—the valve or crystal on one hand, and 
the selenium cell on the other—are not strictly com- 
parable. But the only really important difference lies 
in the lengths of the waves. Roughly, the radio- 
frequency waves commonly employed are one thousand 
‘million times as long as those operative in photo- 
telephony. This difference is of great importance in 
relation to the mode of propagation. Wireless waves 
at present in use are so long that they turn readily 
round corners, so that not only does the earth’s curvature 
impose no serious limitation of range, but broadcasting 
in all directions is possible and, indeed, inevitable. 
Light waves, on the other hand, are for practical 
purposes propagated rectilinearly ; consequently photo- 
telephony can never be expected to attain a very great 
tange. It has, however, the compensating feature that 
by its directiveness it implies not only secrecy of com- 
munication but non-interference by simultaneous 
transmissions, without the necessity of tuning as in 
radio-telephony. It is true that selenium is more 
sensitive to red light than to other colours, and is 
therefore somewhat selective as regards frequency, but 
the suppression of the other colours is not called for, 
and would, in fact, be a disadvantage. 
The photophone,as distinct from the photo-telephone, 
has several other applications. Two may be briefly 
indicated. The modulated light from the transmitter 
can be focussed into a narrow line upon a uniformly 
moving kinematograph film upon which, after develop- 
ment, there appears a band of varying opacity corre- 
sponding to the light fluctuations, and, therefore, to 
the speech orother sounds used for modulating purposes. 
The same film, on being run at the same speed between 
a source of light and a selenium cell with a suitable 
optical arrangement, gives a reasonably good re- 
production of the original sounds. With sufficient 
amplification the results can be heard proceeding from 
a loud-speaking telephone. The application of this 
form of gramophone to the problem of synchronised 
pictures and sounds is obvious, and has been described 
in an earlier article.2 Many workers in various countries 
are now concentrating their attention upon perfecting 
a system of this kind, and there is no reason to suppose 
that realisation will be long delayed. 
The speech currents controlled by selenium under 
the action of the modulated light from a photophone 
transmitter compare favourably in accuracy of form 
with those obtained by means of a carbon microphone. 
The photophone as a whole—.e. the transmitter and 
receiver together regarded as one unit—can thus be 
used as a substitute for the microphone in cases where 
stricter accuracy in electrical sound transmission is 
desired. This necessity has arisen in acute form in 
connexion with radio-telephony, in which the radio- 
frequency oscillations have to be modulated in the 
transmitting valve as nearly as possible in accordance 
with the sounds it is desired to transmit. A photo- 
phone has been used successfully at the Manchester 
broadcasting station for this purpose, and for some 
months those who listen to this station have been 
receiving the results of what can quite fairly be described 
as a remarkable sequence of occurrences. A singer 
sings, and the aerial vibrations thus created fall upon 
a diaphragm. This is forced also into vibration and 
imparts its motion to a small mirror, which in turn 
deflects a beam of light so that more or less of it reaches 
a selenium cell. By its photo-electric property the 
cell controls an electric current so feeble that it has 
to be amplified by thermionic valves in seyeral suc- 
cessive stages before it is intense enough to modulate 
efficiently the radio-frequency oscillations in the trans- 
mitting valve. Thence the modulated wave travels 
through the ether to the receiving aerial ; here, perhaps, 
it undergoes one or more high-frequency magnifications, 
and then the modulations are detected by a crystal or 
valve. Then there may be several low-frequency 
amplifications before, eventually, the fluctuating 
current actuates a telephone diaphragm causing it to 
re-create those aerial vibrations which we hear. When 
we bear all this in mind our attitude is not that of 
criticism of the defects of reproduction, buc rather that 
of amazement that it so closely resembles the original. 
® Nature, vol. ro8, p. 276 (1921). 
Recent Experiments in Aerial Surveying by Vertical Photographs.' 
By Prof. B. Metvitt Jones and Major J. C. Grirritus. 
Il. 
COMPILATION OF THE Mosaics. 
a Vay compilation of the mosaics presents consider- 
able difficulties unless approached in a systematic 
manner, for, although individual prints fit well together, 
1 Continued from p. 709. 
NO. 27G6, VOL. 111 | 
there are always some slight errors which tend to 
accumulate, unless special precautions are taken to 
prevent this occurring. 
We begin the compilation by laying out each strip 
of photographic prints separately, paying special atten- 
tion to the joins between successive prints. Slight 
