January 13, 192 1] 



NATURE 



643 



The Physical and Optical Societies' Exhibition. 



THE Physical and Optical Societies held tlieir 

 ili-venih annual exhibition of scientific apparatus 

 at South Kensington on January 5-6. There was a 

 record attendance. The demonstrations and dis- 

 courses were unusually attractive, a considerable 

 number of visitors failing in their attempts to attend 

 the discourses because of the overcrowded condition 

 of the large lecture theatre. Sir W. H. Bragg gave 

 an interesting lecture on " Sounds in Nature," and 

 .Mr. C. K. Darling showed by means of beautiful 

 fxperiments some little-known surface-tension pheno- 

 mena. On behalf of Prof. Archibald Barr, Dr. Mor- 

 rison gave two addresses on the optophone, the 

 instrument exhibited being the result of much patient 

 research .md development work by Messrs. Barr and 

 Stroud. In the optophone a selenium bridge is 

 exposed to successions of sets of light pulsations, 

 which vary accordin^j to the forms of letters as these 

 are i)assed over in traversing a line of printed type, 

 each letter being indicated in a suitably connected 

 telephone by a characteristic succession of single notes 

 and chords. Printed letters are thus translated by the 

 c)pti>|>hone into a sound alpfcibet which can be readily 

 learned. Miss Mary Jameson, a blind girl, who 

 attended the exhibition, read ordinary type at about 

 ten words per minute, but when undisturbed her 

 normal rate is about twenty-five words per minute. 

 Many blind people were present. 



Thermionic tubes and associated appliances were 

 much in evidence. Prof. C. L. Fortescue and Dr. 

 Brvan gave a very instructive demonstration of well- 

 Unown circuit arrangements having all the parts 

 'Xposed to view. The "heterodyne" or "beat" 

 inethf)d of reception was demonstrated, and surprised 

 many visitors because of the clearness of the beat 

 tone, .\nother demonstration involving the use of 

 valves was given by Messrs. Creed and Co. The 

 .tpparalus which was shown received and recordc-d 

 wirele.ss messages at a working speed of 200 words 

 per minute. The record is » punched slip of paper 

 which by means of a printer is transcribed into Roman 

 characters. The research laboratories of the General 

 Electric Co. exhibited a number of valves with a new 

 type of filament. This filament runs at a much lower 

 temperature than the ordinary tungsten filament in 

 the valves in general u.se at present. The increased 

 strength resulting from lower temperature permits the 

 use of finer filaments, the watts for heating these 

 being onlv one-twentieth to one-fortieth of usual 

 values. Thus it is possible to work a six-valve 

 .implifior with a current consumption of half an 

 rimpere. Characteristic curves for these valves are 

 very similar to those for existing types in use for 

 wireless telegraphy. 



In the optics section, Mr. Aldis exhibited a rom- 

 r)aratively cheap but very perfect projector specially 

 suited for projecting pond-life on the screen. With 

 live specimens and a magnification of 300, the ali- 

 mentary canal and internal organs of manv speci- 

 mens were shown with great clearness. There should 

 lie a good future for this projector for educational 

 tiurposcs. Messrs. .Adam Hileer demonstrated a 

 little-known application of the Fabry-Perot interfero- 

 tnrlrr. A beam of white light is rause<! to traverse 



successively two plates of air, each with silvered 

 faces, a system of fringes being obtained whenever 

 the differences of path occasioned by each of the 

 plates bear to each other a simple relation. If, then, 

 a Fabry-Perot ^talon is placed in series with a Fabry- 

 Perot interferometer (the air plate in the dtalon 

 having a constant thickness and in the interferometer 

 a variable thickness) a system of white-light bands is 

 produced every time the distance between the silvered 

 surfaces of the interferometer mirrors is either a 

 multiple or sub-multiple of the distance between the 

 plates of the 6talon. Messrs. Hilger also demon- 

 strated the Zeeman effect with a wave-length spectro- 

 meter and a Lummer-Ciehrcke parallel plate. Messrs. 

 R. \V. Paul and F. Twyman dehionstrated by the 

 use of a Hilger-type interferometer the distribution 

 of temperature around a hot body. The convection 

 currents produced in air by electrically heated wires 

 were beautifully shown. 



Cathode-ray workers were interested in a 12,000-volt 

 direct-current generator set shown in operation by 

 Messrs. Evershed and V'ignoles. .\ cathode-ray tube 

 was placed in circuit and some of the possibilities of 

 the method of investigating rapid changes of current 

 were demonstrated. 



The general display of .all exhibits was particularly 

 good, and great credit is due to the forty-eight instru- 

 ment-making firms who exhibited for the excellence 

 of their manufactures and for the care taken iiT 

 arranging and explaining the purposes of their instru- 

 ments. The finish of electrical instruments was of a 

 very high order — much higher than last year, when 

 many mass-production instruments were shown. In 

 many cases it was gratifying to find that elegance 

 of production was accompanied by a surprising robust, 

 ness. As an example a simple galvanometer which 

 appeared to be very good for schools was shown by 

 Messrs. Gambrell Bros. ; this has a self-locking 

 device when it is not supported on a table, and, in 

 consequence, it withstands extraordinary shocks in 

 transit. The Cambridge and Paul Instrument Co. 

 showed for the first time a modification of the 

 Einthoven string galvanometer having six strings, 

 the deflections of which are recorded on a moving 

 band of photographic paper. This tyiw of galvano- 

 meter was first used in sound-ranging in France, and 

 was of great service during the war. An excellent 

 amplifier for cable work (but it should have manv 

 other applications) was exhibited by Mr. H. W. 

 Sullivan; the amplifier is a selenium-cell relay which 

 is acted upon by a light beam from a galvanometer; 

 a magnification of 10,000 was obtained. .As usual, 

 the Weston Instrument Co. had an excellent display 

 of meters and parts of meters which only requirecl 

 inspection for one to understand why a Weston instru- 

 ment is alwavs trustworthy. 



The optical section of the exhibition attracted 

 increased attention because of the position of the 

 optical trade as a kev industry. The exhibits were 

 certainly of a high class, and it is hoped that the 

 efforts of the optical industrv to establish itself firmly 

 and to make its pro<lucls inferior to none will receive 

 sunport from the large number of visitors who admired 

 the exhibits. 



The Headmasters' Association. 



THERE was a large muster at the annual meeting 

 of the .'\ss<K'iation of Headmasters heUl in the 

 I fuildhall of the City of London on January 5. In 

 his presidential address Mr. J. Talbot, headmaster of 

 the Royal Oraminar .School. Newcastle-upon-Tyne. 

 I'.inillril the new psychology in a sensible way. No 

 2672, VOL. 106] 



one can question the results of psycho-analysis when 

 applied to ca<*'s of neurosis. Slany a soldier owes 

 his recovery from shell-shoik to the skill of men like 

 Dr. Rivers'. Prof. Elliot Smith, and Prof. Pear. But 

 when a smatteror who has merely "read rt Imok," or 

 perhaps only listened to a lectun-, begins to fumble 



