lU 



KNOWLEDGE. 



March. 1911. 



exhausted, but decreasiug rapidly from the time when the 

 accumulator is switched on for live or ten minutes. Lamps 

 used in this way may easily give rise to errors of ten per cent, 

 or more, an amount which cannot often be considered 

 negligible. The question as to the " primary " standard to 

 be adopted for photographic purposes is still quite undecided, 

 and is, probably, the most important problem to be considered 

 by the next International Congress of Ph<itography. 



THE THEORY OE THE IRIS DIAFHR.^GM.— Mr. 

 C. E. Lan-Da\is recently read before the Optical Society a 

 paper in which he worked out the geometrical theory of the 

 iris diaphragm, showing the conditions which will produce 

 the maximum aperture, both for complete closing and for 

 closing to a known aperture. The aims in designing an iris 

 diaphragm are to obtain : — 



The greatest apei'ture consistent with the smallest opening, 



A long scale with equal division, 



Light weight. 



Ease in working, 



Ereedom from faihue, and 



Reasonable cost. 

 The factors which can be varied are the position of the 

 pin in the fixed plate, the position of the pin in the slotted 

 plate, width of the leaf, the length of the leaf, the nnuiher of 

 the leaves, and the angle of the slots. For the greatest 

 possible aperture the pins should be placed at the opposite 

 extremities of the leaves, and twenty-five leaves are required. 

 The maximum aperture obtainable is eighty-two per cent, of 

 the total diameter of the iris, and no increase in the number 

 of leaves will give a greater opening. If, however, the 

 smallest apertures are not required, the number of leaves may 

 be reduced or a greater aperture obtained. 



The length of the scale is governed by the position of the 

 pin. while by altering the angle of the slot, the division of the 

 scale can be made more or less even without altering the 

 total length of the scale ; but the angle of the slot can only 

 depart slightly from the radial direction without introducing 

 uncert.iinty in setting. 



THE EXHIBITION OE THI-: ROYAL PHOTO- 

 GRAPHIC SOCIETY.— The Royal Photographic Society 

 will hold its Annual Exhibition during the month of May, 

 at Prince's Skating Club, Knightsbridge. 



In addition to the usual Pictorial Section. Section 2 will be 

 devoted to photographs selected for their technical excellence : 

 Section 3 to colour photographs, including transparencies by 

 the screen-plate processes; Section 4 to Natural History 

 photographs ; and Section 5 to Scientilic photographs. 



As Sections 4 and 5 are of special interest to the readers 

 of this column it may be desirable to give the names of the 

 members of the selecting committees of these Sections, who 

 will both select the pictures to be hung and award medals. 

 The judges for Section 4 dealing with Natural History will 

 be Messrs. W. Earren, F. Martin-Duncan, O. G. Pike and 

 Dr. Erancis Ward ; while those for Section 5, which consists 

 of photographs showing the application of photography to 

 scientific objects, such as photo-micrographs, spectra, astro- 

 nomical phott)graphs, and radiograms, will be Dr, W". Deane 

 Butcher (X-ray work), Messrs. Chapman Jones. J. W. 

 Ogilvy (photo-micrography), W. Shackleton (astronomy), 

 Major-General Waterhouse, and the writer. 



Pictures must be sent in by Monday, April i4th. Entry 

 Eorms and Regulations can be obtained from the Secretary, 

 35, Russell Square, W'.C. 



PHYSICS. 



By A. C, G. Egerton, B.Sc. 



GENERAL. — There is so much that is of very great 

 interest in the physical papers that have recently been 

 published that it is quite diihcult to choose those which are of 

 the greatest general interest and demonstrate the advance 

 that is being made in the various branches of physics. Lord 

 Rayleigh has been investigating the perception of colour. Sir 

 J. J. Thomson is [jerfecting a means of analysing the nature 

 of a highly rarefied gas which he came upon in his studies of 



positi\i' rays. Sir William Ramsay and Dr. R. W. Gray have 

 accomplished a wonderful feat in weighing the emanation of 

 radium. These are a few instances of the great activity that 

 prevails in every branch of physics. 



NATURE OF THE X-RAYS.— There is considerable 

 controversy at the present time about the nature of X-rays. 

 Professor Bragg advocates the view that the X-ray and the 

 7-ray emitted by radioactive substances, consist of an electron 

 bearing with it a circumscribed positive electrification 

 sufficient to neutralise the negative charge — this has been 

 termed a " neutral pair." The older view, which originated 

 with Sir George Stokes, was that the X-ray was an ether pulse 

 travelling outwards and with ever-extending front. On this 

 idea, as the distance from the source increased, so the energy 

 at any point on the front of this spasmodic wave decreased. 

 The X-rays and 7-rays give rise to secondary rays when they 

 fall upon substances ; and these rays consist of 1^ particles. 

 /i particles are electrons such as radium ejects ; they are 

 precisely similar, though more rapidly moving, to the kathode 

 rays obtained when the electric discharge is passed through 

 vacuum tubes. Now it is found that the X-rays and especially 

 7-rays are able to liberate these secondary rays and to ionise 

 the gas through which they pass at considerable distances 

 from the source. This fact is difficult to explain on the pulse 

 theory, but it is what would be expected if the X or 7-ray is a 

 discrete particle moving at a great speed, comparable with 

 that of light. 



Professor Bragg discussed this most interesting question in 

 his lecture on the kinetic theory of a fourth state of matter at 

 the Royal Institution, on January 27th. He pointed out a 

 similarity between the molecular kinetic theory, which deals 

 with the collisions and motion of molecules, and the motion of 

 the radiations from the radioactive elements — a./i. 7 rays — and 

 of X-rays. There are strikingdift'erences and striking similarities. 

 Molecules, when they collide with each other, do not inter- 

 penetrate, but rebound when their spheres of influence touch. 

 The a particle or other ray is able, in virtue of its small size 

 and extreme velocity, to penetrate the atom it meets. The 

 effect of the penetrated atom on the a or ^* particle is such that 

 knowledge of the internal arrangement of the centre of force 

 inside the atom may be obtained. 



The resemblance between the motion of molecules and that 

 of these rays is not only that they both move in straight lines 

 and are darting about to and fro with great speed and are 

 undergoing frequent encounters, but also it appears that in 

 each case the sum of the energies of the colliding particles is 

 the same before and after impact. An a particle, for instance, 

 collides with an atom — the particle is moving with such 

 velocity that although the atom abstracts part of the energy of 

 the particle, yet the a particle (whether it be the same or 

 another) emerges in the same direction, and with a kinetic 

 energy equal to that of the original particle, less the energy 

 absorbed by the atom. The latter quantity is proportional to 

 the square root of the atomic weight — a remarkable fact, 

 l-^ventuallv the a particle is so slowed down by collisions that 

 it is easily deflected from its course on an encounter with an 

 atom and moves in a more random fashion, splitting up the 

 molecules of the gas it encounters into electrically charged 

 " ions."' 



It appears that the secondary rays produced by X-rays are 

 formed according to the same law of equal energy before and 

 after impact, and it would follow, thus, that their nature is 

 probably discrete ; and further, since they are undcflected by 

 magnetic and electric fields, that they are neutral particles. 



But ultra-violet light has the pow-er of setting free fi particles 

 from metals on which it impinges, and it has long been 

 considered to consist of waves of short length. 



Professor Bragg's theory cannot yet be considered to be 

 (luite complete. 



THE ULTRA-VIOLET RAYS.— Professor R. W. Wood 

 has come across a curious phenomenon which he has recently 

 described in the Philosophical Magazine. He finds that if 

 a condenser spark is passed between aluminium terminals and 

 the direct light is shaded from the recipient of the light, 



