January. 1911. 



KNOWLEDGE. 



33 



Figure 1. 



A \ie\v from the great Bridge o 

 in Oporto. 



exposures too short ; but some of the results obtained proved 

 of considerable interest. 



Two t\'pical examples are reproduced herewith. The first 

 is taken from the great bridge o\er 

 the Douro, in Oporto, and shows 

 well one of the most striking features 

 of all these photographs — the in- 

 tense blackness of the blue skw 

 (See Figure 1.) Even the faintest 

 trace of cloud stands out at once 

 against the blue sky when observed 

 by infra-red light, so that it would 

 seem possible that the method might 

 be of considerable use to the meteor- 

 ologist. The shadows in infra-red 

 light are extraordinarily deep, show- 

 ing no detail at all if the sky be clear. 

 because the blue skylight contains so 

 little light of long wave length. 



The second photograph, which is 

 taken in the Avenue Liberdad, in 

 Lisbon, shows the extraordinary 

 appearance of foliage under this 

 infra-rei. light. The chlorophyll 

 has but che slightest absorption for 

 it, so that the foliage on the trees 

 appears white, while in the left- 

 hand bottom corner we have the 

 curious phenomenon of a white 

 palm 1 (See Figure 2.) 



Incidentally, it is interesting to 

 remember how few coloured sub- 

 stances do absorb infra-red light 

 between seven thousand and eight 

 thousand .A.U. The only two dyes 

 which I know to absorb completeK- 

 in this region are Naphthol Green 

 and the new Hoechst dye, Filter 

 Blue-Green. The new Badische 



Anthraquinone dyes absorb to seven thousand fi\e hundred 

 but transmit light of greater wa\'e length ; nearly all other 

 blue and green dyes transmit 

 red light of wave length seven 

 thousand or under. 



AX "EXPLOSION" THE- 

 ORY OF THE LATENT 

 IMAGE. — In a letter to The 

 Brifisli Journal of Photo- 

 ^rapliy. Mr. F. F. Rci.'.vick 

 makes a suggestion for a new 

 theory of the latent image, though 

 various facts pointing in the 

 same direction have been pub- 

 lished at intervals. This theory 

 is based on the observation of 

 Dr. \V. Scheffer, that a silver 

 bromide grain on exposure, 

 \iolently throws off a part of its 

 substance, rupturing the surround- 

 ing gelatine in its passage. 



Mr. Renwick suggests that, in 

 an emulsion, the silver bromide 

 grains are wrapped round by a 

 tangled meshwork of gelatine, 

 and can only be attacked either 

 through the extremely minute 

 channels left, or by diffusion 

 through the substance of the 

 gelatine skeleton. Now, if Dr. 

 Scheffer's observations be 

 accepted, then in the neighbour- 

 hood of an exposed grain the 

 densely tangled network is broken 

 through, and channels of relatively 

 large size, giving far readier access 



C. L. Kenitt-llt Mas. 



n the Donro 



J-ri'irt ail t nj r tt-na piiOiO^ttJl^ 



for the developer to the silver bromide grains, are formed. 

 In some respects this theory seems :j be more satisfactory 



th;in any hitherto held. It has general';,- been assumed that 

 the commence. nent of development 

 depended upo; 'he provision of a 

 nucleus, upon \> ;?h the silver pro- 

 duced by the interaction of silver 

 bromide with the vsducer could 

 precipitate. This remiiins probable, 

 but a difficulty was that, in this case, 

 once fogging from an u.iexposed 

 developer had commenced, it should 

 have proceeded at the normal rate. 

 This does not seem to be the case ; 

 if measurements be taken . of the 

 increase of fog with time of develop- 

 ment for an unexposed plate, it is 

 found that the function obtained is 

 similar to that given by an exposed 

 plate, but with a much lower velocity 

 constant. 



This is accounted for at once if 

 the exposed grains have literally 

 become " exposed "' to the attack of 

 the developer, by blasting passages 

 through their surrounding network. 

 This explosion theory is also 

 \aluable in that it enables one to 

 give a meaning to the " ripening " 

 of an emulsion. A "' ripened " grain 

 would be one which was in the most 

 explosive state : that is, in which 

 the crystallisation occurring during 

 cooking had reached the limit of 

 stable equilibrium, so that any 

 further access of energy would 

 result in its disintegration. 



The theory is certainly fascinating 

 in its possibilities, though it will have 



to face much criticism, especially from a consideration of the 



destruction of the latent image by oxidisers, and of the 



desensitising action of some 

 metaUic salts, when added to 

 the emulsion. 



PHYSICS. 



By W. D. Eggar, M.A. 



THE CAVENDISH LABOR- 

 .ATORY. — On Saturday, Novem- 

 ber 1 ith, a large and distinguished 

 company assembled at " the 

 Cavendish " to do honour to Sir 

 J. J. Thomson, whose twenty-five 

 years of office as Professor of 

 Experimental Physics have been 

 signalized by the publication of 

 the ■■ History of the Cavendish 

 Laboratory." The Vice-Chancel- 

 lor presided, and Dr. Glazebrook, 

 of the National Physical Labora- 

 tory, presented a specially bound 

 copy to his old friend and former 

 chief. Few institutions possess 

 a more distinguished record than 

 this Cambridge Laboratory, young 

 as it is. James Clerk Maxwell 

 was appointed as first professor 

 to the newly constituted chair in 

 1871. The Laboratory, built 

 under his directions, was opened 

 in 1874. When Maxwell died, 

 in 1879, Lord Rayleigh succeeded 

 to the post, which he held until 

 1885. [^j. J.jThomson, then a very 



t,h h- C. E. KviiiuHi .l/<-,-: 



Figure 2. 

 The Avenue Liberdad in Li.sbon. 



