ii6 



NATURE 



[September 23, 1920 



printed on to a sensitised gelatine film on the metal 

 cylinder A, and the parts unaffected by light 

 washed away to leave the metal exposed, so that 

 contact may be made with the brush B. 



One of the best known practical developments of 

 photo-telegraphy was made by Dr. Korn some 

 twelve years ago. In his apparatus the original 

 picture was photographed on to a transparent film, 

 which was then wrapped round the cylinder A — 

 in this case made of glass. Instead of the contact 

 brush B, a fine pencil of light was used, which, 

 after passing through the film, fell on a sensitive 

 selenium cell and thus modulated the current 

 flowing out to the lines in accordance with the 

 opacity of the film. At the receiver the incoming 

 current was passed through a special form of 

 galvanometer — practically a double-thread Ein- 

 thoven galvanometer^so that a light aluminium 

 shutter was moved aside to a greater or less 

 extent, depending upon the strength of the current. 

 A beam of light was thus controlled in intensity 

 before falling upon a sensitive film attached to the 

 receiving drum D. The path traced out by this 

 beam followed a spiral track on the surface of D, 

 as in the case of the brush C of the arrangement 

 of Fig. I, and similar governing or synchronising 

 arrangements were of course required. 



A photographic recorder of this type possesses 

 considerable advantages over the simple electro- 

 lytic arrangement outlined at first, and enables 

 much more accurate work to be accomplished. It 

 is obviously not the only possible arrangement to 

 obtain the desired result, but, although differing in 

 detail, the other varieties that have been used 

 operate upon similar principles. 



The chief disadvantage of the selenium-cell 

 modulator at the transmitting station is to be 

 found in its inertia to rapid changes of illumina- 

 tion, with the consequent limitation of the speed 

 of operation. In the practical apparatus a compen- 

 sation arrangement was employed to increase the 

 speed of transmission, but better results would be 

 possible with a light sensitive material having less 

 lag. Some of the more recently discovered 

 sensitive materials may prove better in this 

 respect. 

 . An ingenious alternative transmitter has recently 



been put to practical test. The photographic print 

 of the original picture at the transmitter is made 

 in gelatine or similar material, giving a relief 

 print when "developed." Instead of passing a 

 metallic contact over such a print, a needle is used 

 in such a manner that difference in relief varies the 

 resistance of a microphonic contact, and thus 

 controls the line current. The reproduction at the 

 receiving station is effected with a photographic 

 apparatus of the type described above. 



From the transmission of pictures by wire, it is 

 but a relatively small step to their transmission 

 by radio. In the earlier stages of wireless develop- 

 ment attempts were made in- this direction with 

 more or less success, but the irregularities of the 

 coherer, which was at that time the only receiver 

 capable of controlling sufficient local energy to 

 operate the recording apparatus, prevented the 

 practical development of the method. With the 

 modern three-electrode valve receivers and relays 

 this diflficulty is removed, and their use at the 

 transmitting station as continuous-wave generators 

 is also advantageous. Considerable developments 

 on these lines may therefore be expected in the 

 near future. In particular the last described 

 transmitter, with microphonic control, should be 

 especially easily adaptable to radio work by using 

 any one of the well-known radio-telephone modu- 

 lation methods. 



During the last few months some considerable 

 interest has been aroused by a method of photo- 

 graph transmission applicable to either wire, cable, 

 or wireless signalling. Strictly speaking, however, 

 this method is not of the same class as the above, 

 in which the picture itself controls the actual sig- 

 nalling currents, as the transmission is effected by 

 a series of code letters, words, or numbers, sent as 

 an ordinary telegraphic message between the two 

 stations. By this method the original picture is 

 divided up into a number of small units, and a 

 code letter used to designate the condition of light 

 or shade in that unit, thus enabling a complete 

 code message to be built up to represent the whole 

 picture. At the receiving station the decoding 

 may be effected in an obvious way, or a 

 mechanical decoding machine may be employed to 

 reproduce the picture. 



The Structure of the Atom.^ 

 By C. G. Darwin. 



III.— T/ie Nucleus. 

 T N the previous articles we'discussed the nucleus 

 -»• mainly in connection with the idea of atomic 

 number. We shall now return to the characters 

 of the nucleus itself; but before doing so it is 

 necessary to say something about the atom as a 

 whole. Comparatively little is known about the 

 electrons surrounding the nucleus — it is not even 

 universally agreed whether they are at rest or in 

 motion — but a successful beginning has been made 

 by. the Bohr spectrum theory, which applies 



1 Continued froii p. 83 



NO. 2656, VOL. 106] 



mainly to the specially simple case of hydrogen. 

 On the same lines Sommerfeld has also had some 

 success with the X-ray spectra. It is quite certain, 

 as in all questions of atomics, that the laws of 

 classical dynamics do not hold, and the principal 

 method in research at present consists in a 

 judicious mixture of these laws with the quant\im 

 theory. This latter theory is definitely contra- 

 dictory not only to the laws of mechanics, but also 

 to almost any conceivable modification of them, 

 and its chief justification, an entirely adequate 

 one, is the astounding success with which it has 



