November. 1910. 



KNOWLEDGE. 



457 



similar to that presented to the eye when the sun shines 

 through a Ught haze. Owing to this effect, shadows, which by 

 infra-red light appear solidly black, are. in ultra-\'iolet light, 

 almost entirely absent, the diffused light from the skv prevent- 

 ing the formation of clear shadows. 



The reflecting power of natural substances for ultra-violet 

 light is, of course, very different from that which they possess 

 for visual light. 



Silver has only about the same reflecting power as glass, 

 and would appear like anthracite coal, while Chinese white 

 absorbs the rays more powerfully than printer's ink. White 

 garden flowers also absorb the ultra-violet light strongly. 



Photographing the surface of the moon. Professor Wood 

 found a large deposit near Aristarchus which appears quite 

 black by the ultra-violet light, though it is not distinguishable 

 by \isual light. 



The method seems to hold out some promise for the 

 in\'estigation of the petrolog\- of our satellite. 



METHODS OF IMPROVIXG UNS.ATISF.ACTORV 

 TECHNICAL RESULT S.— While some scientific 

 photographs leave nothing to be desired, the technical cjuality 

 of others is so poor that they pro\e less satisfactory for 

 purposes of demonstration than their author might desire. 

 Apart from actual bad work, there are two chief causes 

 tending to produce inferior results ; — 



(1.) Sufficient exposure cannot be given, or is very 

 difficult to gi\e ; e.g., in photographing objects in 

 motion. 



(2.) The contrast in the subject is insufficient. 

 The results obtained in both these cases are susceptible of 

 improvement by after treatment, and I have found that the 

 following methods ai'e satisfactory : — 



(1.) If the negative is so badly under-exposed that after 

 development to the limit only a ghost is obtained, ordinary 

 methods of intensification are not of nmch use. After thorough 

 washing, the plate should be completely bleached by leaving 

 in a solution of mercuric chloride until the whole deposit has 

 become quite white to the back. The negative is now washed 

 and dried, care being taken that it does not come into contact 

 with developer, hypo, or ammonia solution. After dr\-ing. the 

 back of the glass is covered with a black varnish, or with 

 ' Photopake,' or Indian Ink. The photograph will now appear 

 as a good positive, which can be put in front of a camera 

 and copied. 



(2.) If the contrast in the subject is insufficient much may 

 be done in the case of coloured subjects by using colour filters 

 with appropriate plates to increase the contrast, but some- 

 times this is useless. A case in point was supplied by the 

 photography of the A and B lines in the solar spectrum with 

 considerable dispersion and a high sun. The resulting nega- 

 tive, while admirable for purposes of measurement, gave quite 

 insuflicient contrast for printing, even when intensified. I 

 therefore made a contact positive upon a slow lantern plate, 

 and developed the positive with a physical developer, con- 

 sisting of an acid solution of metol to which a small quantity 

 of silver nitrate had been added. By this means the blackness 

 of the deposit can be increased to any desired extent. A 

 contact negative was then made from the positive, in the same 

 manner, and from this excellent prints were easily obtained. 



A PHOTOGRAPHIC PROCESS BV FERMENTATION. 

 — Dr. Leo Jacobsohn, in a paper in PJiotographische 

 Rundschau, describes some experiments which he has made 

 on the peptonic fermentation of gelatine by the gastric juice, 

 or by solutions of pepsin. Gelatine coated upon glass plates, 

 and containing bichromate, was exposed to light and then 

 subjected to the action of pepsin solutions, with the result 

 that theunexposedgelatine was dissolved, while that which had 

 been exposed to light proved incapable of digestion. 



Gelatine which was hardened by the usual hardening agents, 

 such as alum or formaline, proved as subject to attack by 

 pepsin as unhardened gelatine. 



Dr. Jacobsohn has employed pepsin for " developing " pig- 



mented gelatine prints, instead of hot water, but it would seem 

 doubtful if there is any real use for the process. 



Further investigations upon the same lines, however, might 

 prove useful as a means of enquiring into the structure of 

 gelatine, and the alterations produced in its molecular structure 

 by \arious reagents. 



PHYSICS. 



By W. D. EgGar, M.A. 



RADIU.M STANDARDS AND NOMENCLATURE.— In 

 Nature of October 6th. Professor Rutherford gives an 

 interesting summary of the discussions which took place at 

 Brussels in September, at the International Congress of 

 Radiologj' and Electricity. -At the opening meeting he himself 

 read a report on the desirability of establishing an international 

 radium standard. He had compared by the X-ray method 

 several European standards and had found differences in 

 some cases amounting to twenty per cent. It is possible to 

 measure with considerable accuracy such magnitudes as the 

 heating effect, the rate of production of helium, and the rate 

 of emission of a and j8 particles. But the value of such 

 determinations depends on the accuracy of the radium standard 

 used in expressing the results. A powerful Conmiittee of 

 representative workers was appointed to report on the best 

 method of fixing an international standard. This committee 

 reconuuended. and the congress adopted their recommendation, 

 that Madame Curie, herself a member of the Committee, should 

 prepare a radium standard containing about twenty milli- 

 grammes of radium. This standard will cost about ,^,500, and 

 will become the property of the International Committee. It 

 will probably be kept suitably at Paris. 



It has been suggested that the name Curie should be used 

 to express the quantity or mass of radium emanation in 

 equilibrium with one gramme of pure radium. The amount 

 in equilibrium with one milligramme of radium would then be 

 called one millicurie. The (juestion of names of radioactive 

 products was discussed informally at the Congress. The 

 present system seems to be fairly satisfactory, and capable of 

 extension. For instance, if radium C is found to consist of 

 se\eral constituents, these might be called Ci Ca, etc. But the 

 giving of fancy names by individual workers is regarded as 

 undesirable. 



A NEW OPTICAL GRATING.— Prof. R. W. Wood, 

 whose reputation as an experimenter extends far beyond the 

 United States, delivered the " Thomas Young Oration " to the 

 Optical Society on September 29th. In this he described a 

 new form of grating, occupying a position intermediate between 

 the ordinary diffraction grating and the echelon. It is ruled 

 on gold deposited on copper, using a crystal of carborundum, 

 which possesses an advantage over diamond in having straight 

 sides meeting at an angle of 120'. The gold is compressed so 

 as to form ridges and hollows with highly polished and almost 

 perfectly flat sides. The spacing is coarse. These gratings 

 are specially suitable for heat waves, and are much more 

 efficient than prisms of rock-salt. Prof. Wood has given the 

 name echelette to this form of grating. 



ZOOLOGY. 



By Professor J. .\rthur Thomson, M.A. 



BARNACLES AND SNAKES.— One of the most curious 

 of zoological pictures, mediaeval at first glance, is that given 

 by Dr. Willey (in Spolia Zeylaiiica. May 1910), of a sea-snake, 

 Hydrus platurus. bearing a bunch of barnacles on the end of 

 its tail. The barnacles are of two kinds iLepas anserifera 

 and Coiicltodernia liuiiteri) and form a dense group. The 

 attachment of barnacles to the skin of sea-snakes has long 

 been known, but the case figured is \ery quaint. The 

 specimen was brought alive to the Colombo Museum. Dr. 

 Willey writes : " The barnacles are not ectoparasites, as they 



