Al'KIL, 1911. 



KNOWLEDGE. 



155 



atoms of mercury ; so that the particles grow rapidly in size, 

 and from the known specific gravities of the various amalgams 

 the increase in the diameter of the particles can be calculated. 

 Eight such intensifications produce an enlargement of over 

 seven diameters, and even particles which were at first com- 

 pletely ultra-microscopic may be thus rendered measureable. 



The method appears to be capable of giving results of 

 considerable accuracy, probably of the order of five per cent, 

 and the results obtained were satisfactory concordant. 

 They showed that in films, where the particles were 

 too small to produce any visible colour, only a faint 

 blue opalescence being visible, the diameter of the particles 

 ranged from -10 to -12 microns. If the films were yellow 

 the diameter was -13 to •14 microns; -15 to -17 gave 

 orange or brown films, and in one case pink, while -17 to -18 

 gave purple tones, diameters above -IS corresponding to 

 brown-blacks and greys. 



These diameters are somewhat less than those found by 

 Schaum and Schloemann for their grains. 



The refractive index of gelatine was measured by a number 

 of methods, the most successful being the finding of a liquid 

 mixture of known refractive index in which the edge of the 

 film disappeared ; the result obtained was 1 • 53. Now it will 

 be seen that the diameters given for the various particles 

 nearly correspond to the half wave-lengths in gelatine of the 

 light whose absorption would produce the colour stated, thus 

 the wave-length of the limit of the visible spectrum. -4 microns 

 in air, will become -26 microns in gelatine, the half of which, 

 •13, corresponds well with the diameter of the particles which 

 commenced to show a yellow colour. 



Mr. Chapman Jones gave some remarkable instances of the 

 accuracy of the relation found between the colour of the 

 deposit and the size of the particles ; he had examined 

 images of mixed colours and had always found that such 

 images contained particles of at least two different diameters, 

 corresponding with the different colours. 



He had also found that where the particles were of the same 

 size, but were dispersed to different extents, the colours were 

 the same, thus showing that Zsigmondy's suggestion that the 

 colour depends on the dispersion of the particles cannot be 

 substantiated for silver particles in gelatine films. 



These results obtained by Mr. Chapman Jones are entirely 

 different from those of Schaum and Schloemann, who experi- 

 mented with a chlor'de emulsion developed with various 

 developers, and also with gelatine films containing silver 

 nitrate printed out, and who investigated the subject by means 

 of the ultra-microscope. 



The conclusions to which these workers came were : — 



(1) In coloured images obtained under their conditions the 

 ground was coloured and the particles of considerable size, 

 and black, in opposition to the results of Kirchner and 

 Zsigmondy, who found that in gold gelatine preparations the 

 ground was colourless and the particles strongly coloured. 



(21 The colour was unaltered by diluting a coloured film 

 with gelatine and re-coating and drying. The colour was also 

 little altered "by intensification, bright red becoming dark red, 

 bright green, dark green, yellow passing through orange to red, 

 and blue becoming deep violet. The colour was also little 

 altered by reduction, simply becoming less saturated. 



(3) The colour is associated with the thickness of the layer 

 of grains, as was found by Kirchner for Lippmann films, 

 rather than with the size of the particles. 



It is to be hoped that Mr. Chapman Jones' valuable paper 

 "ill lead to further work upon the subject, which may result 

 in a reconciliation of these apparently conflicting results. 



THE EFFECT OF COLOUR FILTERS UPON THE 

 DEFINITION OF A LENS.— If colour filters be used with 

 a lens, it is clear that considerable attention should be paid to 

 the optical accuracy of those filters, so that they do not intro- 

 duce aberrations which may aflect the definition of the image. 

 Apart from the accuracy of the glass itself, distortion may be 

 produced in colour filters in the course of their manufacture 

 in several ways. 



In the first place, if the filters are prepared by coating 

 coloured gelatine upon the glass, then when this gelatine dries 



it will contract and bend the glass ; also, when the filter is 

 cemented with Canada Balsam, too rapid drying or drying at 

 uneven temperatures will distort the filter ; while, finally, if 

 pressure is exercised upon a thin filter in its cell, the filter may 

 easily be permanently strained. If these strains were 

 symmetrical they would be of small importance, as they would 

 simply produce a lens of slight positive or negative power, and 

 so, to a small extent, change the focal length of the lens with 

 which they are used. But generally they are either in one 

 direction only, or are much greater in one direction than in 

 the other, and so produce a cylindrical lens, which introduces 

 astigmatism. The effect of such aberration naturally becomes 

 much greater as lenses of longer focal length are used, the 

 effect varying as the square of the focal length of the lens, so 

 that a filter which would be perfectly satisfactory on a hand 

 camera lens of six inches focus, would be with a telephoto 

 combination quite useless. With medium and high power 

 telephoto lenses only filters of the highest optical accuracy 

 can be used. 



This point must be carefully borne in mind, in view of the 

 recent introduction of what may be termed semi-telephoto 

 lenses, such as the Busch Bis-Telar, which naturally require 

 that a filter should be far more accurate than would be 

 assumed to be necessary for its diameter. 



The aberrations of filters can be minimised by making them 

 of as thick glass as possible, having regard to its optical 

 accur.acy. and for filters of the very highest quality it is usual 

 for the two glasses to be about five millimetres in thickness. 



PHY.SIC.S. 



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



GENERAL. — Among the events of the last month, mention 

 should be made that the electromotive force of the Weston 

 Normal Cell, made up according to specification, has been 

 accurately found to be 1^0183 international volts at 20"C. ; 

 standard cells will in future be compared with such Weston 

 Normal Cells at the National Physical Laboratory. The 

 Weston cell consists of an H -shaped vessel, the one limb of 

 which contains mercury and mercurous sulphate, the other 

 containing cadmium and cadmium sulphate. Its electro- 

 motive force is very nearly independent of temperatiu'e over 

 the usual range of working temperatures. The E.M.F. of the 

 Clark Cell varies with temperature, while its E.M.F. is greater 

 than the Weston cell, the former having a zinc negative pole 

 instead of cadmium. 



Professor Perrin gave a most lucid exposition of his investi- 

 gations on Brownian Movement at the Royal Institution on 

 February 24th. During his lecture he showed, by means of 

 cinematograph photographs, the movement that colloid 

 particles undergo when an emulsion of gum and water is pre- 

 pared and examined under the microscope : their rotation, 

 their collision, their random path, and their attraction and 

 repulsion from electrified surfaces were beautifully illustrated. 

 By an able investigation of the motion of these minute colloid 

 particles and assuming the motion of molecules to be of the 

 same character. Professor Perrin has succeeded in calculating 

 in several ways the number of molecules per cubic centimetre 

 and mass of the atom of hydrogen, the numbers agreeing well 

 with those determined by other methods, such as the period of 

 change of radium, the charge carried by the atom of hydrogen, 

 the polarization of light and consequent blueness of the sky, 

 and the values obtained by measuring the viscosity of gases for 

 the average distance traversed bj- the molecules between 

 collisions or their " Mean Free Path." The number of 

 molecules per cubic centimetre is nearly twenty-eight trillions. 



THE VISCOSITY OF GASES.— In the Proceedings of 

 the Royal Society there have appeared lately several interest- 

 ing communications by Dr. A. O. Rankine. There is much 

 that is pleasing about a simple apparatus : the viscosity of gases 

 has hitherto been a somewhat troublesome co-efficient to 

 determine, but Dr. Rankine is able to find the viscosity of 

 Xenon, which can only be obtained in small quantity, with com- 

 parative ease, by means of his apparatus. The apparatus 



