JlT.V, 1911. 



KNOWLEDGE. 



283 



ANTARCTIC BIRDS.— The Scottish National Exhibition, 

 now open at Glasgow, contains a section illustrating the 

 Antarctic Expedition of the " Scotia." There is a good display 

 of the birds and seals of the .Antarctic, shown in artistic 

 representations of their natural surroundings. One of the cases 

 represents a scene in the South Orkneys, and conveys a \'ivid 

 impression of Antarctic bird life. Five species of penguins, 

 with eggs and young, are represented. Notably there are 

 three Emperor penguins, standing in solitary state on an ice 

 floe, one of them trumpeting a love-song to his mate. .Among 

 the smaller birds, the most prominent are black-throated or 

 adelia penguins. A pair are seen hurrying back to their nests 

 after ha\ing had an excursion to sea in search of food. .An 

 anxious mother is seen feeding her young, while another pair 

 sit expectant alongside an unhatched egg. There are also the 

 gentoo penguins, with red-marked bills and white-striped 

 heads : the macaroni penguin of the golden crest, and the 

 ringed penguin. 



Twelve other species of .Antarctic birds are represented. 

 These include the blue-eyed shag, in its nest of seaweed ; the 

 Dominican gull, resembling the black-back gull of our own 

 waters ; and the fierce skuas fighting over the body of a 

 penguin. The only land bird of the .Antarctic, white and in 

 form like a chicken, is also shown. Then there are the petrels. 

 — the giant petrel, sitting on its nest of stones ; the silver 

 petrel, the .Antarctic petrel, the blue petrel, and the snowy 

 petrel, whose presence is indicative of the ice pack. There is 

 a fine specimen of the mottled black and white Cape pigeon, 

 whose eggs the " Scotia " naturalists were the first to discover, 

 although the bird has been known for at least three hundred 

 years. 



PHOTOGR.M'HY. 



Hy C. E. Kenneth Mees. D.Sc, F.C.S., F.R.P.S. 



THE FOGGING POWER OF DEVELOPERS.— A paper 

 on this subject appears in the June number of the 

 Pliotojiraphic Journal. It is. of course, well known that 

 developers differ in the amount of fog which they produce on 

 unexposed plates, but this appears to be the first systematic 

 attempt to compare the fogging power of different developers 

 and to distinguish the conditions which tend to produce fog in 

 development. In order to compare the fogging power of 

 different developing solutions, it is necessary to measure the 

 amount of fog which they will produce in the time necessary 

 to produce a given degree of contrast. 



In the development of an exposed plate, the rate at whicli 

 density grows is usually expressed by what is called the 

 " velocity constant," which quantity may be denoted by the 

 symbol K. (See "' Knowledge """ Notes," May. 1911.) The 

 value of K for a given plate and developer is easily determined 

 and if we can find some equally simple means of determining 

 and expressing the rate at which fog grows in an unexposed 

 plate, it is obvious that a comparison of the two constants 

 should gi\e an idea of the fogging power of the de%elopcr 

 used. 



For example, if the rate of growth of fog on an unexposed 

 plate is exactly equal to the rate at which the density grows in 

 exposed parts of the same plate, it is obvious that no image 

 can result, the fogging power being so great that we can only 

 obtain a uniform deposit over exposed and unexposed portions 

 alike. A developer giving such a result will be useless, and it 

 is in all cases necessary that the velocity of the fogging action 

 shall be less than that of development. 



If we express the fogging velocity by the symbol F. just as 

 we express the developing velocity by the symbol K, then the 

 ratio of F to K can be taken as a measure of the fogging 

 power of the developer, and therefore we may measure F and 

 K separately, and then, dividing F by K, obtain a quantity 

 which can be used as a measure of the fogging power. 



In the production of measurable degrees of fog on 

 unexposed plates very considerable duration of development 

 was necessary and, as any oxidation of the dexeloper aflccted 

 the results, the plates were developed in a tube which was 

 quite full of developer, a rubber cork fitted with a stop-cock 



being pushed in until the developer spurted out from the 

 stop-cock, which was then closed. In order to avoid 

 temperature errors, the tuV>e was a vacuum-jacketed Dewar 

 tube. 



The developer used was Hydroquiuone and Caustic .Alkali 

 and the results obtained showed that the fogging power of 

 such a developer is mainly due to two causes — oxidation 

 products of the developer itself, and the sodium sulphite used 

 as a preservative. The fogging power of non-oxidised 

 developers not containing sulphite was independent of the 

 amount of alkali present, increase of the alkali increasing the 

 development of both exposed and unexposed bromide in the 

 same proportion, leaving the ratio unaltered. 



The oxidation product of the hydroquiuone developer, 

 quinhydrone, was found to be a powerful fogging agent in the 

 presence of alkali, developing exposed and unexposed bromide 

 at the same rate and conse(|uently developing no image on an 

 exposed plate. Since this compound cannot exist in the 

 presence of sodium sulphite ordinary photographic developers 

 do not give oxidation fog! But sulphite itself is a source of 

 fog, the amount of fog depending on the concentration of the 

 sulphite and rising to a maximum at a low concentration, 

 decreasing again as the concentration is increased. 



The explanation given by the authors for the phenomena 

 described by them is based on a discussion of the reduction 

 potential of developers. 



The reduction potential of a reducer, or the oxidation 

 potential of an oxidiser is the analogy for chemical affinitv of 

 electric potential in electricity. 



On the other hand, the resistance potential of a chemical 

 substance (Cig., silver bromidel may be compared to the " back 

 potential " of an arc lamp. It requires a certam voltage 

 (about forty volts) to enable an arc with carbon poles to burn, 

 and if the potential of the source of electric energy is lower 

 than this, then the arc will not burn. 



If iron poles are used in the arc a higher potential labout 

 sixty-five volts) is required to enable the arc to burn; so that 

 there is a range of voltages (from forty to sixty-five volts) 

 where carbon arcs can burn but iron arcs cannot. 



The authors suggest that the difference between fogging 

 agents and developers is analogous to this difference between 

 the voltages, which will work arc lamps with different poles. 

 .A substance with a low reduction potential (e.g., neutral 

 hydroquiuone) cannot reduce either exposed or unexposed 

 silver bromide. If the reduction potential is higher then 

 exposed silver bromide becomes dexelopable. but the 

 resistance potential of unexposed silver bromide is still too 

 high. All substances which have reduction potentials higher 

 than the resistance potential of exposed silver bromide, but 

 lower than that of unexposed silver bromide will be developers. 

 If the reduction potential of a substance is higher than the 

 resistance potential of unexposed siher bromide (as is the case 

 with alkaline i|uinhydrone, for instance) then it will be a 

 fogging agent. 



Any substance which tends to lower the resistance potential 

 of unexposed silver bromide will increase the fogging power of 

 a developer, and. since the resistance potential of a substance 

 depends upon its insolubility, any substance, such as sulphite, 

 which increases the solubility of silver bromide will increase 

 the fogging power of a developer, while a substance, such as 

 an alkaline bromide, which decreases the solubility will 

 decrease the fogging power. 



The subject would seem to require a good deal more 

 investigation and probably the theory outlined abo\e will 

 require nuich modification, but it is s.atisfactory that some 

 progress is being made with a subject about which so little 

 has hitherto been known. 



PHYSICS. 



By A. C. G. Egi.rton. B.Sc. 



Physicists have had the opportunity of examining tlie fruit 

 of each other's work during the past month. The Physical 

 Society has enjoyed the opportunity of visiting the National 

 Phvsical Laboratory : while the Conversa/iione of the Royal 



