53° 



NA TURE 



[February i, 1906 



tion as carbohydrate and fat. Endogenous metabolism 

 sets a limit to the lowest level of nitrogenous equilibrium 

 attainable, and the proteid necessary to balance this part 

 of the nitrogenous waste is indispensable for the repair of 

 the tissues. Whether the amount exogenouslv katabolised 

 can be entirely dispensed with is at present questionable. 

 I fancy must physiologists would agree that it cannot with 

 safety be wholly dispensed with ; the body would then be 

 working too dangerously near the margin, and in any case 

 where an excess of nitrogenous waste is necessary the 

 call would have then to be made on the tissue proteids. 



Recent researches on digestion of proteids in the alimen- 

 tary canal have shown lhat they are largely broken down 

 into simple substances like ammonia, leucine, tyrosine, and 

 other arnino-acids. This is regarded by Folin as a pre- 

 liminary means ol getting rid of the excess of proteid 

 in. Hh 1 usually ingested; these waste products, according to 

 this view, are taken to the liver, rapidly transformed into 

 urea, and so got rid of. The evidence that they are 

 svnthesised by the cells of the body into " living proteid " 

 is regarded by him as inconclusive and largely teleological. 

 An extensive formation of Voit's "circulating proteid," 

 to be followed immediately by decomposition into urea, is 

 quite as improbable as the corresponding formation and 

 decomposition of Dinger's organised protoplasm. The 

 organism requires in its food only the small amount of 

 nitrogen necessary for endogenous metabolism ; the nitrogen 

 cif the extra proteid is unnecessary, and the organism has 

 at hand an active mechanism for casting it out. 



To attempt to summarise all the points ol detail into 

 which Folin enters is beyond the scope of this article ; all 

 I desire to do is to bring forward the main principle of 

 the new idea. There is, however, one further point of 

 sufficient importance to warrant specific mention, and that 

 is the one related to muscular work. The fact that 

 muscular work does not increase proteid katabolism may 

 be accepted as an approximate truth ; it is not absolutely 

 true; there is a certain increase of nitrogenous waste, but 

 it is insignificant as compared with the enormous and 

 immediate increase of waste carbonaceous products like 

 carbon dioxide that are discharged when muscles are 

 thrown into action. 



If current views on the nature of proteid katabolism are 

 correct, this fact is very difficult to explain, but it becomes 

 intelligible if proteid katabolism, in so far as its nitrogen 

 is concerned, is independent of the oxidations which give 

 rise to heat or to the energy which is converted into work. 

 " Whether severe work will have an effect on the endo- 

 genous metabolism cannot be shown by investigating urea 

 excretion; determinations of creatinine and 'neutral 

 sulphur ' are necessary for a study of that question 

 (Folin). 



One of the benefits such papers as those of Folin confer 

 is that new ideas of this kind suggest fresh work to others, 

 and it can hardly be doubted that in the future physiological 

 literature will contain many papers criticising and supple- 

 menting the tlienries and facts which Folin has brought 

 forward. Already one of these has appeared in the current 

 issue of the journal of Physiology (Noel Paton, vol. xxxiii., 

 p 1, 1905). In this Dr. Noel Paton on the whole agrees 

 with Folin concerning the dual nature of proteid meta- 

 bolism. He, however, differs from him in certain points 

 of detail. lie finds in the dog, for instance, that creatinine 

 excretion is not so constant a quantity as in man. He 

 also doubts whether it is possible to draw any hard and 

 fast line between endogenous and exogenous metabolism, 

 and that urea may be a final product of both. He ex- 

 plains some of Folin's results by variations in the activity 

 of the liver, for it is in this organ that ammonia com- 

 pounds and the like are transformed into urea. A study of 

 various diets upon the flow of bile (which may be taken 

 as an index of hepatic activity) shows that proteid diet 

 specially stimulates the metabolic processes in the liver. 

 Hence on a diet which is poor in proteid the hepatic 

 action may lie sluggish, and will therefore fail to convert 

 a large quantity of waste nitrogen into urea, while on a 

 diet rich in proteid the conversion must be much more 

 complete. As with the nitrogen, so with the sulphur, the 

 amount of which is completely oxidised must be deter- 

 mined by the activity of the changes in the liver. 



Such, then, is a brief summary of some of the recent 



work in connection with these most important problems. 

 We can hardly doubt that the steps made are in the direc- 

 tion of progress of knowledge, but it is as yet too early 

 to prophesy where they will ultimately lead us. 



W. D. H. 



PHOTOGRAPHY IN NATURAL COLOURS. 



A N exhibition of photographs in which the aim of the 

 photographer has been to imitate the colours of the 

 objects represented is now open at the offices of the Britisli 

 Journal of Photography, 24 Wellington Street, Strand, and 

 will remain open until the beginning of March. The speci- 

 mens are all direct photographs in the sense that they have 

 been produced by photographic printing, and not in printing 

 presses from blocks or plates. The editors of the British 

 Journal of Photography must be congratulated in that they 

 have succeeded in bringing together a more representative 

 collection than has ever been on view before. 



The first glance that one instinctively takes round a 

 room immediately on entering it produces a feeling quite 

 different from that experienced on giving a momentary and 

 general look round in a small gallery of paintings. In the 

 latter case there is an impression of completeness in the 

 work that gives satisfaction, whether or not this is main- 

 tained when the pictures are more carefully examined; but 

 here there is a sense of a want of finish, an impression of 

 experiment or incompleteness, as if those who made the 

 pictures had left off before they had got the effect they 

 sought to get. Perhaps others will not experience the 

 same feeling, but it was very marked in the case of the 

 writer, and, so far as the origin of it could ]>> traced, it 

 appeared to be due to a general crudeness of colour, or 

 the predominance of one certain colour over the whole 

 picture, or an indecision of outline that was evidently not 

 intentional. Some examples suffer in one way and some 

 in another; a few are quite satisfying, and must be very 

 excellent if not perfect, but they are not in a sufficient 

 proportion to affect the general impression. 



It will hardly require technical knowledge to convince 

 tin- visitor that the personality of the photographer has a 

 great deal to do with the result. The more skilful the 

 worker the better the photograph, that is, the more true 

 are the colours and the fewer the errors of manipulation 

 in all wavs. As the skill of the worker has so much to do 

 with the result, it is impossible to decide as to the merits 

 of the various methods. Strictly speaking, it is not possible 

 to determine the value of any of the photographs, for in 

 no case is the original put by the side of it. Who would 

 ever dream of attempting to judge the merits of a copy 

 except by comparing it with the thing copied? Yet the 

 writer has never seen or heard of a demonstration of the 

 possibilities of a method of colour photography by an 

 exhibition of a coloured object and its photograph side by- 

 side. 



The effect of the personal element, or, in other words, the 

 varying skill (or perhaps the varying luck) of different 

 workers, is very clearly shown in the examples of the 

 same process by different persons, or where an optically 

 inferior method gives a better result. As an example of 

 this last we would refer to Nos. 12 and 33, both apparentlj 

 from the same group of fruit. &c, and both made of three 

 superimposed films. In No. 33 the films are not cemented 

 together, yet this picture is brighter than the other. 



The only example of the immediate production of the 

 colour of the incident light, and in this case the colour 

 is not pigmentarv but due to interference and visible only 

 at a certain angle, is a very successful spectrum by the 

 Lippmann process contributed by Mr. E. Senior. With two 

 or three exceptions, the rest of the exhibits are three-colour 

 prints. The fundamental principles are the same in all. 

 Three-colour records are made by photographing the object 

 through coloured media, getting the red, green, and blue 

 of the object separately recorded. From these three nega- 

 tives suitably coloured prints are made and brought 

 together. In the Joly process, two specimens of which are 

 lent In Mr. E. J. Wall, the three colours are arranged in 

 series of fine parallel lines, and it is necessary to get so fai 

 away that these lines are indistinguishable, otherwise they 

 are annoying to the spectator. The starch-grain method of 



NO. 1892, VOL. 73] 



