Fkbkuarv. 1911. 



KNOWLEDGE. 



77 



film sufficient for the purposes of ordinary chemical analysis. 



If the quantity of these unattackable grains varies with the 

 exposure it would seem probable that the amount of metallic 

 silver required to produce a given light absorption would vary 

 with the exposure also, and it is known that such variation 

 nmst be very small if it occurs at all. 



A most interesting experiment of Dr. Scheffer's would seem 

 to suggest that the " filaments " can scarcely be produced 

 during exposure. A small quantity of emulsion was placed on 

 a slide and a most powerful beam of light passed through it 

 by a Zeiss projection apparatus and microscope, the image 

 being projected on to a screen. 



Under these conditions the grains could actually be 

 observed to decompose under the action of light, shooting out 

 the " filaments " or clouds of nebulous matter, and going black 

 with growths, apparently of metallic siher, upon them. The 

 amount of light required to produce this effect is very great, 

 however, and there seems to be every reason for doubt as to 

 whether an\- actual decomposition of the grain is produced by 

 ordinary exposures. 



Dr. Schefter also described the investigations which he had 

 made as to the distribution of the image in the film of the 

 ordinary dry plate. The great difficulty in such work is the 

 preparation of sufficiently thin sections, since for first-class 

 results the sections should not be thicker than i m (^cnuth mm). 

 By the aid of a special microtome of his own construction. 

 Dr. Scheffer has been able to attain to most excellent results. 

 and one photograph, showing not only the Zenker's laminae in 

 a Lippmann plate, but the actual grains composing them, was 

 a marvellous testimony to the perfection of Dr. Schefter's 

 methods. 



Most of the results obtained from sections confirm those of 

 previous investigators, but the studies of the action of various 

 " reducers " are of special interest, and make very clear the 

 known differences between the various oxidising agents 

 employed to reduce the density of photographic negatives. 



DEVELOPMENT WITH HVDROOUINONE. — The 



chemical reactions involved in development are generally 

 complicated, and the oxidation products of the organic 

 developers are especially obscure. 



One of the simplest of the organic developers is, of course, 

 hydroquinone, which is oxidised to quinone by silver bromide. 



Using the ordinary nomenclature, the reaction will be 



CnH4(OHI.. + 2AgBr+2NaOH^C„H<Oo + 2.Ag + 

 2NaBr+2HoO, 



one molecule of hydroquinone producing two atoms of silver. 



It has been shown that this reaction is reversible, quinone 

 and soluble bromide having the power to oxidise a silver 

 image to silver bromide. So that quinone and a soluble 

 bromide will act as a photographic reducer lof density) as has 

 recently been pointed out by MM. Lumiere and Se\'ewetz. 



But the simple equation given above does not by any means 

 express the whole of the reactions taking place in a hydro- 

 quinone developer containing alkali and sulphate. 



Quinone reacts with sulphite, oxidising it to Dithionate and 

 being reduced to Hydroquinone thus : — 



Na2SO:^ + C,iH40.2+2H.O = Na.>SoO„-|-2NaOH + C,iH4(OH)., 



Moreo\er, the addition of an alkali to quinone reduces some 

 of it to hydroquinone, apparently being oxidised to peroxide, 

 since the ether extract gives the characteristic blue colour 

 with acid bichromates. So that both the sulphite and the alkali 

 regenerate the reducing agent from its oxidation product, and 

 the development reaction becomes far more complicated than 

 would at first appear. 



PHYSICS. 



By \V. D. Eggar, M.A. 



\\'.-\VES \'ERSi'S K.WS. — A good many years ago 

 Professor C. V. Boys took a series of photographs of flying 

 bullets. In these a prominent feature was the wave from the 

 bow of the bullet, as from the bow of a steamer. It was a 



wave of compressed air, made instantaneously visible to the 

 eye of the camera by the spark which took the photograph. 

 In fact, it was a wa\e of sound. Following up this discovery, 

 Professor R. W. Wood took a series of photographs of sound 

 waves, issuing from a central point. The sound was that of 

 an electric spark, and the photograph.^ were taken by the light 

 of another spark, following close upon the heels of the noisy 

 spark. The interval between the two sparks could not be 

 regulated with any accuracy, but a large number of photo- 

 graphs were taken, which are pubhshed in the form of lantern 

 slides, and appear also in Wood's " Physical Optics." Thej' 

 illustrate many of the main features of wave-motion, and are 

 exceedingly useful in explaining the wave-theory of light. In 

 a paper read to the Physical Society on "Cusped Waves of 

 Light and the Theory of the Rainbow," Mr. W. B. Morton 

 alludes to the tendency at the present time to ba:e the 

 teaching of Optics on the conception of the wave, rather than 

 that of the ray. This tendency was further exemplified at the 

 annual meeting of the Public School Science Masters' Associa- 

 tion, at which Mr. J. Talbot gave a demonstration of his use of 

 the ripple-tank. This is a large flat glass tank with water in it 

 to a depth of about half an inch. It is supported at the 

 height of an ordinary table above the ground on which is 

 placed an arc light, screened from the class. A translucent 

 screen tilted above the tank on the side of the class recei\es 

 the shadows of the ripples made in the surface of the water. 

 The velocity of the ripples can be altered by altering the 

 depth of the water, and this can be done by flat glass plates. 

 Hence, many of the features of reflection and refraction can 

 be demonstrated by the medium of surface ripples. The 

 method forms a most useful addition to the equipment of a 

 teacher of Optics. At the same meeting, Mr. C. F. Mott 

 exhibited his apparatus for teaching Optics by the method of 

 rays. In place of the usual pins employed in elementary 

 practical Optics, Mr. Mott uses a beam of light coming through 

 a pair of narrow slits, fixed at the ends of a groove cut in a 

 block of wood. The conception of rays is, in the opinion of 

 some, the natural introduction to the subject, and if it be right 

 to follow historical order in the presentation of physical 

 knowledge, rays would undoubtedly come before waves. 

 However, as the protagonists agreed, the two methods are 

 supplementary rather than antagonistic ; and it must be borne 

 in mind that the teacher of science has sometimes as his aim 

 the stimulation of interest in physical phenomena, at other 

 times the training of the youthful mind in methods of precision. 



THE RADIO-BALANCE.— In the December number of 

 the Proceedings of the Physical Society there is an important 

 paper by Professor Callendar describing his radio-balance. 

 This instrument is devised for the absolute measurement of 

 the heat of radiation, and is specially applicable for that of 

 Radium and its emanation. The heat imparted by radiation 

 to a small copper disc or cup is directly compensated by the 

 heat absorption due to a Peltier effect created in an iron- 

 constantan thermo-couple connected to the disc, and to a 

 variable source of electric current. The Peltier co-efficient 

 and the value of the compensating current being accurately 

 determined, the heat of radiation can be measured with 

 considerable accuracy, when an exact balance between the two 

 effects is obtained. 



ZOOLOGY. 



By Professor J. Arthur Thomson. 



NON-DIGESTIBILITY OF FAT BY PROTOZOA.— 

 W. Staniewicz calls attention to a curious point, that the 

 Protozoa have never learned to digest fat. .All Metazoa have 

 this power, but Protozoa have not. Experiments with Para- 

 moecium, Stentor, and some other common Infusorians show 

 that fat may be ingested, but it is not digested. It is not a 

 natural part of a Protozoon's food. The fat sometimes found 

 in natural conditions within the cell of a Protozoon seems to 

 be due to the transformation of proteids or carbohydrates. 



MUDDY TASTE IN FRESHWATER FISHES.— It is 

 well known that carp and tench and eels and some other 

 freshwater fishes are apt to be tainted with a peculiarly 



