320 



KNOWLRDGi:. 



Aif.rsT, 1912. 



rt'ccntly polislird. llu- .iiiiomiii nf lij;lit reflected iiorin.illy was 

 foiiiul to be four per cent. Hut it w.is to the loss due to 

 absorption of lit,'ht by the k'jiss coinposiiiK the lens that the 

 experiments wrrc mainly directed, and it was thought that it 

 might be possible from the results obtained to establish a 

 general empirical formula, by which the transtnitting power 

 of a lens might be estimated merely from a knowledge of the 

 number of glass-.iir surfaces in the lens, and the axial 

 thickness of the glass from which it was made. 



The method employed was to compare photometrically the 

 intensities of two beams of light: in the one case when the 

 photographic lens to be tested is inserted iu the path of one 

 of them, and in the other when the lens is withdrawn. The 

 source of light was a Nernst lamp, the filament of which was 

 focused upon a small circular opening placed at the principal 

 focus of an achromatic telescope objective of about eighty 

 centimetres focus. This gives a parallel beam of light which 

 falls upon one side of the diffusing screen of a Lummcr- Brodhun 

 photometer. The other side of this screen is illuminated by 

 means of a glow-lamp enclosed iu a light-tight box. In order 

 to counteract the yellowness of this glow lamp, a circular disc 

 provided with openings carrying a number of tinted glasses 

 which are able to be rotated in front of the lamp is provided, 

 and by this means the colours of the two light sources can be 

 matched. When taking readings, the lens to be tested is 

 placed on a support, and an auxiliary lens (employed to 

 obtain a patch of light of a convenient size) moved along in 

 its holder until a circular patch of light of a useful size is 

 obtained on the diffusing screen of the photometer. The dis- 

 tance of the comparison lamp (glow lamp) is then adjusted until 

 the field of view seen through the photometer is symmetrically 

 illuminated. Several readings are taken and the mean dis- 

 tance of lamp from the screen noted. The lens to be tested 

 is then removed and a similar set of readings taken with the 

 auxiliary lens in position only : whose effect may be neglected, 

 since it is present in both cases. From results obtained, 

 tables have been made showing the percentage transmission 

 of light for twenty-four objectives tested. These tables are 

 arranged under four headings according as the number of 

 glass-air surfaces are four, six, eight, or ten. The author also 

 points out that the percentage transmission in any particular 

 case can be found approximately by the following rule. For 

 each glass-air surface allow for the loss due to reflection 5-22 

 per cent, of the light incident on that surface, and for the loss 

 due to absorption allow 2-4 per cent, of the light incident 

 upon the lens for each centimetre of the axial thickness of the 

 glass composing the lens. The following is given of a case in 

 point : — Consider a lens having eight glass-air surfaces in 

 which the total axial thickness is 2-8 centimetres. At each 

 glass-air surface 5-22 per cent, of the light is reflected and 

 94-78 per cent, only transmitted. Therefore, if reflection 

 only were the cause of the loss of light, the amount transmitted 

 would equal (•9478)" of the incident light, or 65- 1 per cent. But 

 there is the loss due to absorption as well, which in this case 

 amounts to " 2 • 8, the thickness in centimetres, X 2 • 4 per cent., 

 the loss due to absorption of the light incident upon the lens." 

 and this amounts to 6-7 per cent. The final value of the 

 transmitted light, therefore, amounts to only 58-4 per cent., 

 I.e., 65-1 — 6-7. The author states that the observed value 

 was 58 per cent., the error, -4, being due to the difference 

 between the percentage transmission calculated by this rule 

 and that actually observed. The paper is a most valuable 

 one, and will be found well worth careful consideration by 

 those interested in the subject. 



III.— PROOF PLATES.— An interesting exhibit of test 

 plates used in checking the polishing of lens surfaces and 

 made to an accuracy of 1'4000 of a millimetre was exhibited 

 by Messrs. J. H. Dallmeyer, Ltd., of Denzil Road, Neasden ; 

 those used in testing plane surfaces showing broad bands 

 instead of wide rings. The same firm also showed the 

 various stages through which their well-known stigmatic lens 

 passes during the course of manufacture, as well as finished 

 examples of their series II. stigmatics, which really comprise 

 four lenses in one. for they can be used complete as a rapid 

 anastigmat, and the single components are corrected for use 



.iliiiie. e.icli giving ;i picture on a larger scale than the com- 

 plete lens. And, further, as the latter can be used success- 

 fully on a larger pl.ite, it becomes a wide-angle lens as well. 

 There were shewn the latest fixed focus telephoto lenses work- 

 ing with large apertures, so permitting of instantaneous 

 exposures and giving images from two to five times that given 

 by ordinary lenses at the same extension, e.f>., the Adon, a small 

 complete telephoto lens covering plates up to fifteen inches by 

 twelve inches, and giving magnifications of from two to four, 

 six, ten and higher diameters. An iris diaphragm shutter of 

 maximum opening, showing the greatest aperture it is possible 

 to obtain consistent with closing in this form, after a paper 

 communicated to the Optical Society by Cyril B. Lan-Davis. 

 F.R.P.S., was on view. The maximum opening theoretically 

 obtainable is 88 per cent.; with this form. 76-5 per cent, of the 

 total diameter is utilised. The same firm also exhibited a 

 series of large prisms, in one of which the effect of uneven 

 cooling was distinctly visible as striae or veins. On Saturday, 

 the 22nd of June, a number of members of the Convention 

 visited Messrs. Dallmeyer's works, and were greatly interested 

 in seeing the various processes through which photographic 

 and other lenses pass, and the methods employed in the 

 manufacture of optical instruments. The party included 

 Messrs. F. B. Vinycomb, Alex Mackenzie, B. H. Parker. 

 J. Hartlev Perks, James Robert Milne, James Grundv, 

 J. W. Ogil'vy. A. E. Charlton, F. \V. Edridge-Green, C. B. D. 

 Macklow, and Mrs. C. Macklow. 



IvXFOSURE TABLE FOR AUGUST.— The calculations 

 are made with the actinograph for plates of speed 200 

 H and D, the subject a near one, and lens aperture F.16. 



Remarks. — If the subject be a general open landscape, take half 

 the exposures given here. 



I'HVSICS. 



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



THE CHEMICAL EFFECT OF X-RAYS.— The ultra- 

 violet rays are well-known to have considerable effect in 

 causing chemical changes to take place ; there are many 

 instances of isomenic charge : of oxidation and reduction 

 which take place more rapidh- under the influence of these 

 rays. It is interesting to find, therefore, that the X-rays have 

 similar eftect. Dr. Russ and Mr. Colwell have found that 

 starch solutions are slowly converted into dextrin under the 

 action of the X-rays. Kernbaum has found that X-rays do 

 not decompose water, like the rays of radium are found to do. 

 producing hydrogen and oxygen, so that the effect of the 

 -X-rays on the starch molecule must be a direct one. 



Sir William Ramsay has confirmed the result that niton (or 

 radium emanation) produces neon and helium in presence of 

 water, both by direct experimental evidence from the analysis 

 of gases pumped ofl" from water exposed to a dose of niton for 

 two years and also by the observational evidence that Bath 

 waters, which are radio-active, contain a very large quantity of 

 neon compared with the quantity of argon present ; the gases 

 from the waters of Bath contain a considerable quantity of 

 helium also, but an even larger (luantity of neon. 



It is an exceedingly interesting fact that the decomposition 

 of niton in solution gives niton and helium, while in solid 

 minerals, and so on, it only gives helium. Is the niton a kind 

 of polymerised helium? or is it produced by the action of 



