284 



KNOWLEDGE. 



July, 1911. 



Society, on May 10th. included exhibits of very great interest. 

 Further than this, many illustrious men from foreign countries 

 have honoured their scientific cousins with their presence in 

 this country, strengthening thereby the absolute internationality 

 of science. Among these one may mention Professor Svante 

 .\rrhenius, Professor Richards and Professor Wood. It will 

 be well this month to note shortly a few of the contrivances 

 which have thr6wn light on the subjects recently investigated. 



Professor Wood, in a lecture to the Royal Institution, on 

 Friday, May 19th, described many experiments which he has 

 made on ultra-violet light. Using (juartz lenses and silvering 

 them with a uniform coating of silver, he has been able to 

 photograph objects by ultra-violet light: the silver absorbs all 

 the visible light. In such photographs the sky is very bright : the 

 ultra-violet light gets scattered by the atmosphere to a very great 

 degree, so much so that no shadows appear. Many quite white 

 bodies, such as Chinese white and most garden flowers, do not 

 reflect ultra-violet light, and so appear black. Professor Wood 

 has photographed the moon by ultra-\iolet light, and obtained 

 patches which show e.xceptional absorption. To exhibit this, 

 he had reconr.se to the ingenious method of projecting two 

 photographs on to a screen so that the two images were com- 

 pletely in register and superposed. The one photograph is 

 taken b\- ultra-violet light, the other by ordinary light. A red 

 filter is placed in front of one photograph and a green in front 

 of the other, so that any difference in the two is at once 

 detected by a preponderance of red or green light on the 

 screen, instead of a complete neutralisation of the two com- 

 plementary colours. With infra-red light there is no scattering 

 by the sky, which appears quite black in photographs taken 

 through a screen of cobalt glass and certain organic dyes 

 which cut off all light of shorter wave-length than A 6S00, 

 But the leaves of trees reflect this deep red light almost 

 completely. These effects were dealt with more completely by 

 Dr. Mees in " Knowledge " a few months back. 



Professor Wood has been able to isolate short ultra-\iolet 

 waves from a magnesium spark by a system of quartz lenses. 

 The centre portion of one of the lenses is stopped out 

 altogether, while the distance apart of the lenses is so 

 arranged that the ultra-violet light alone can pass through 

 the outer portion of the second lens, the more refrangible 

 rays being stopped from passing by the centre blackened 

 portion of the lens. There is another way of isolating the 

 ultra-violet waves alone, but this latter method only obtains 

 the more refrangible of these rays : those which can penetrate 

 glass. A screen made of nitrosodimethylaniline combined 

 with a cobalt glass attains this object. 



Messrs. Wratten and Wainwright supply such light filters. 

 Their list includes colour filters for many purposes, such 

 as the isolation of the green mercury line ; a screen which 

 in combination with a mercury lamp gives an absolutely 

 monochromatic beam. The balancing of the colours in the 

 tricolour screens is very perfect and they should have many 

 applications ; the researcher, the lecturer, the photographer 

 and the microscopist will find many uses for them. 



Lord Rayleigh has pointed out that transparent objects are 

 only visible when they are illuminated unequally. Kaufmann 

 has devised a simple [nethod of illustrating this point. .A 

 funnel of white cardboard is illuminated inside by a glow lamp 

 and vertically up the axis of the funnel is fixed a glass rod. 

 Looking through a slit in the side of the funnel the rod is 

 invisible until it or the lamp is shifted to one side of the axis of 

 the funnel. A transparent object immersed in a fluid of the 

 same refractive index is invisible; for instance, the end of a 

 glass rod is invisible when dipped in Canada balsam (or. better, 

 in a solution of eight volumes of chloral hydrate in one volume 

 of glycerine) while, when withdrawn, the end appears to melt. 



When using interferometers it is often very difficult to 

 obtain sufficient magnification of the bands of light obtained 

 without too great a loss of light. Lord Rayleigh some years 

 ago devised a most ingenious way of getting over this 

 difficulty. He constructed a telescope with a cylindrical lens 

 as an eyepiece and a lens tilted at an angle to the vertical 

 plane as object glass : in this way he obtained a telescope 

 magnifying in only one direction. 



It is only possible to compare intensities of light definitely 



when the light is of the same colour, i.e., of the same wave 

 length. The comparison of differently coloured sources 

 should be made by means of a spectrophotometer — a spectro- 

 meter with a Nicol prism which, on rotation, cuts oft" more or 

 less of the light of any particular wave length. The numerical 

 value of the total intensity of any light source may be obtained 

 from the monochromatic intensities by allotting to each colour 

 a co-efficient, and then summing up the intensities obtained 

 thus for all the colours. M. Thovert has recently worked 

 on these lines and has selected the distinct vision of form as 

 the basis from which to deduce the value of these co-efficients. 

 This method is preferable to that based on obtaining a colour 

 sensibility curve. In the one method a grating spectroscope 

 is used, and a monochromatic ray is isolated by means of a 

 slit, and the luminosity regulated by suitable means ; a 

 pattern is placed in the path of the light at the limit of 

 distinct vksion. and the wave lengths found for any particular 

 degree of illumination for which the pattern is just \isible. 

 The other method is dependent on finding the extreme wave 

 lengths visible when the light from a spectrum is shielded by 

 grey-tinted glass of different depths. There are many 

 problems of great interest in photometry of light of different 

 colours, the subject of colour blindness being intimately 

 connected with it. We must return to the question in another 

 issue. 



The measurement of resistance of a mercury column has 

 been employed by T. R. Merton to calibrate capillary tubes, 

 always a troublesome proceeding when done by measuring 

 the change in length of a pellet of mercury. The ends of the 

 capillary tube communicate with two mercury cups which are 

 connected to one arm of a Wheatstone's bridge. The mean 



., l + -'dr 

 raduis r is given by r"= 7— where 1 is the length of the 



capillary, f the found resistance and d a correction for the 

 stream lines at the ends of the capillary, k the specific 

 conductivity of mercury. It has occurred to the writer that 

 measurements of volume in gas burettes might be made by 

 inserting an iron resistance wire down the burette inside so 

 that the mercury uncovers more or less of the wire as it is 

 raised or lowered, the change in resistance of the wire being 

 accurately measured. 



Professor Theodore Richards has employed a method of 

 electrical contacts for measuring volumes very accurately. As 

 the mercury is lowered so it can be set to just make or break 

 an electrical contact consisting of a fine platinum point. He 

 has used the method to determine dift'erences in the com- 

 pressibilities of liquids. 



Mr. E. H. Rayner has recently described an ingenious 

 arrangement for measuring small thicknesses and displace- 

 ments with far greater sensitiveness than the micrometer will 

 attain to. It consists of three conical feet, two in one 

 plane and the other half-way between them, but slightly 

 out of the plane, so that they rest in a hole, slot, and 

 plane at the corners of a very obtuse angled triangle. 

 The three feet are affixed at the upper end, to a lever 

 carrying a concave mirror focussing a spot of light 

 into a screen. Any slight shift of the third foot, the 

 one that lies slightly out of the plane, causes a slight 

 rotation of the lever, which is magnified considerably by the 

 spot of light. The author has employed the apparatus to 

 measure the expansion of short bars of quartz, only fifteen 

 centimetres long ; also uses are found for it in measuring 

 thicknesses of mica, paper and foil ; or, again, as an adjunct to 

 a chemical balance, to obtain a first approximation to the 

 weight. In the Latter case the three legs of the tilting table 

 are affixed to the end of three rods fitted at their other ends 

 into a block of metal. The centre rod, on being bent by 

 affixing a weight to it, will then tilt the table proportionally to 

 the weight applied and the position of the weight on the rod. 



In Messrs. Paul's recent catalogue there are to be found 

 two instruments of considerable interest which have been 

 designed and placed on the market. The one is a thermoelectric 

 junction and unipivot galvanometer combined with a special 

 compensator patented by .Mr. C. R. Darling. This com- 

 pensator eliminates any error due to the heating of the cold 

 junction, ."^ny change in the temperature of the cold junction 



