December, 1911. 



KNOWLEDGE. 



483 



of Light, that the best definition is obtained when the 

 diameter D of the aperture is related to the distance L 

 from it to the plate, as given by the equation 



D = 2 \/^ L 



where \ is the wave length of the light employed. 



Taking into account the wave length of light of maximum 

 photographic activity. Sir W'illiam Abney gives the formulae 



D 



iVl 



120 



where D and L are measured in inches. It was proved by 

 Lord Rayleigh that when the conditions specified by his 

 formulae are complied with, no further improvement could be 

 obtained. We have before us as we write a photograph 

 taken by means of a plain aperture, in which the conditions 

 set forth above were fulfilled The distance of the plate was 



nine inches, and from our formulae D =t:^ ^^"^ obtam 



ITS inch as the diameter of the aperture that w^ould give the 

 best definition with the plate placed at nine inches from it. 



COLOUR PHOTOGK.APHV UPON PAPER.— However 

 beautiful may be the colour photographs in the form of 

 transparencies upon glass, there is still the feeling among a 

 large number of persons that the only really satisfactory 

 solution of the problem of photography in colours will be 

 found in a process which is applicable to paper. Quite 

 recently the attention of photographers has been drawn to a 

 new paper which has been placed upon the market for the 

 purpose: prepared by Dr. J. H. Smith. The general principle 

 upon which several experimentalists appear to be working is 

 that suggested by Wiener, who considered that the chromo- 

 sensitive surface should be black, and composed of, at least, 

 three physical elements. The writer experimented in this 

 direction some few years ago, employing paper coated with red, 

 yellow and blue dyes. The dyes being fugitive, the prepared 

 paper was found, after exposure under a coloured transparency. 

 to give a fairly good rendering of the object, although in every 

 case the time necessary to obtain the results was \ery pro- 

 longed, amounting to days in the strongest light. To what 

 extent this and other difficulties may have been removed we 

 are unable to say, as we ha\e not had an opportunity of trying 

 any of the recently prepared papers. 



PHYSICS. 



By Alfred C. G. Egertox, B.Sc. 



POSITIVE IONS FROM HOT SUBSTANCES.— These 

 have been the subject of numerous investigations recently. 

 Professor Sir Joseph Thomson found that aluminium phos- 

 phate gave off a large number of positively-charged particles 

 when deposited on a platinum strip which was heated to about 

 lOOO'C. Garrett and also Horton have investigated the effect 

 and have shown that the production of these positively 

 charged ions is intimately associated with the production of 

 small quantities of carbon monoxide — a very commonly 

 occurring substance in vacuum tubes submitted to the electric 

 discharge. 



Professor Richardson has investigated the carriers of 

 positive electricity from strips of platinum coated with 

 alkalis and alkaline earth metals. He finds evidence to show- 

 that these carriers are atoms of the metal carrying a single 

 charge. Even the divalent metals carry only a single 

 positive charge, which is contrary to what might be expected, 

 since these metals are chemically equi\alent to two atoms 

 of hydrogen. The heated platinum itself gives off a certain 

 number of charged ions and Professor Richardson has in- 

 vestigated this thoroughly. He suggests that the considerable 

 discharge of positive particles from aluminium phosphate is 

 due to the impurity of alkah metals it contains. These 

 positive ion discharges are not nearly so marked in effect as 

 the negatively charged particles emitted by the alkaline earth 



oxides. If a small piece of sealing wax be allowed to coat a 

 platinum strip mounted in a vacuum tube so that an electric 

 current can be passed through it, and if there is a difference 

 of potential of 100 volts or thereabouts, between the strip and 

 another electrode in the tube, then on heating the wax. only 

 lime and barium oxide is left which causes a copious discharge 

 of negative corpuscles. Professor J. J. Thomson has utilised 

 these discharges to obtain steady striations in a vacuum tube 

 and by investigating the behaviour of a cathode ray beam 

 from a side tube as it passes transversely through the electric 

 field in various portions of the striated discharge, he has been 

 able to find the magnitude of this electric field, and hence 

 show that the striations are due to the crowding up of 

 corpuscles. The collisions of these with the ions of the gas 

 cause the latter to radiate light. From these areas of 

 collision corpuscles acquiring great velocity are shot off in the 

 direction of the positive electrode and until, by collision with 

 the gas molecules in the tube, their velocity is reduced to such 

 an extent that they can no longer keep on in a straight path, 

 there exists a dark space. Thus a striated discharge comes 

 about ; the number of striations depending on the pressure of 

 the gas in the vacuum tube. 



\\"hen an electric discharge is passed through a gas at low 

 pressure, it has long been questioned whether the gas is 

 electrolysed ; that is, suppose one passed an electric discharge 

 through hydrochloric acid gas, would hydrogen be found in 

 greater concentration at the cathode and chlorine at the 

 anode, as in the electrolysis of the aqueous acid. Professor 

 Thomson many years ago succeeded in obtaining the hydrogen 

 spectrum at the cathode and the chlorine at the anode and on 

 reversing the current the hydrogen and chlorine changed 

 places, the hydrogen still being found at the cathode. How- 

 ever Kayser, the illustrious author of the " Handbuch der 

 Spectroskopie," has asserted that these effects are due to 

 thermal effects, the cathode being always hotter than the 

 anode. Recent experiments at the Cavendish laboratory have 

 apparently shown without doubt that a separation of an 

 electrolytic nature does occur, but that the effect is upset by 

 the mobility of the gaseous ions and consequently thermal 

 effects come in to a great extent. Many organic halogen 

 compounds have been investigated, and the chlorine is some- 

 times found at the anode and sometimes at the cathode, but 

 as a rule e.xactl}- in accordance with its chemical behaviour, 

 either as an anion or cation. In a compound such as methyl 

 sulphide, sulphur would be found at the anode and the carbon 

 monoxide spectrum at the cathode. These results have been 

 obtained by Mr. G. Stead. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A. 



ASSOCIATION OF CRAB AND HYDROID.— Everyone 

 knows that many animals grow upon others in an accidental 

 sort of way, because the young stages landed there. This is a 

 fortuitous epizoic association. Sometimes, however, the 

 association, though remaining entirely external, is by no means 

 fortuitous ; thus many spider-crabs pluck hydroids along with 

 sea-weeds from their surroundings, and plant them on their 

 shells, with the result that they are "masked." Different from 

 this again is a case like the following, recently described by 

 Dr. W. T. Caiman, whose delightful introduction to Crustaceans 

 ('"The Life of Crustacea," Methuen. 1911) should be read by 

 all interested in Natural History. A crab from Christmas 

 Island showed a hydroid polyp, allied to Stylactis, attached 

 like a tassel at the "knee" of each of its legs. This is 

 interesting in several ways. All but two of the polyps were 

 svmmetrically disposed ; the type specimens of the species 

 iMcdacu^ has-ii-elli) to which the crab belongs, although 

 coming from a distant locality, were found to bear a similar or 

 identical hvdroids: the crab is a Xanthid. not one of the spider- 

 crabs in which masking is common ; even the rootwork or 

 hydrorhiza was very precise in its occurrence, following the 

 inter-regional grooves on the carapace. 



