March. 1915. 



KNOWLEDGE. 



87 



or, if the plate be known to be over-exposed, a little of this 

 solution added to the developer previous to the commence- 

 ment of development will in most cases work wonders. The 

 plate should always be allowed to remain in the citrate of 

 soda solution for about half a minute, when it is taken out 

 and replaced without washing in the developer again, and 

 development allowed to proceed until the requisite density 

 is obtained. If, after fixing, the lines or shadows of the 

 negative appear to be at all clogged, the image, after very 

 thorough washing, should be treated with Farmer's reducer, 

 prepared as follows : — 



A. 

 Potassium ferricyanide ... ... 24 grains 



Water ... ... ... ... 1 ounce 



B. 

 Sodium thiosulphate (" hypo ") ... 96 grains 

 Water ... ... ... ... 1 ounce 



The solutions A and B must be mixed together at the 

 moment required for use, as thev do not keep when once 

 added together. If treatment with this reducer diminishes 

 the density of the high lights, then intensification must be 

 resorted to. For this purpose Monchoven's intensifier will 

 be found very suitable, as it gives great density, and is, at 

 the same time, non-staining ; and, moreover, intensification 

 can be repeated if necessary'. The following is the formulae 

 for the solutions : — 



No. 1. 

 Mercuric chloride ... ... ... J ounce 



Potassium bromide ... ... J 



Water ... ... ... ... 20 ounces 



No. 



Silver nitrate 

 Water (distilled) 



i ounce 

 10 ounces 



No. 3. 

 Potassium cyanide (pure)... ... h ounce 



Water (distilled) ... ... ... 10 ounces 



No. 3 is added gradually to No. 2, when a dense precipitate 

 at once forms ; as more of No. 3 is added the precipitate 

 first formed begins to dissolve up, and when only a small 

 quantity remains the solution is ready for use. There must 

 not be any excess of potassium cyanide. This solution is 

 kept in a stoppered bottle, labelled " Silver Cyanide 

 Solution." The negative to be intensified is placed in No. 1 

 until thoroughly bleached ; it is then very thoroughly 

 washed, and then placed in the silver cyanide solution to 

 blacken. The necessity of thorough washing after every 

 operation, and especially after treatment with either 

 " hypo " or mercur\', cannot be too strongly impressed, 

 nothing less than from a quarter to half an hour under a 

 constant stream of water being sufficient. In fact, if it is 

 a subject of importance that we are at work upon, an hour 

 is sometimes occupied in washing ; and in this way we never 

 fail to obtain clean and brilliant negatives suitable for any 

 purpose that may be required of them : certainly in any 

 class of negative work where these agents are employed, 

 the secret of success lies in the thoroughness with which the 

 washing is performed, and when these conditions are fulfilled 

 a negative intensified with mercury is both bright and clean, 

 and of considerable permanence. But the fact remains 

 that for this particular class of work the results are not 

 equal to those obtained upon wet collodion. 



PHYSICS. 



By J. H. Vincent, M.A.. D.Sc, A.R.C.Sc. 



SELENIUM CELLS. — The term " selenium cells " may 

 be suitably confined to photo-electric cells in which one of 

 the electrodes consists of a layer of selenium. Cells of this 

 type were investigated by Minchin. A strip of aluminium 

 is coated with selenium, raised to a temperature of 200° C, 

 and, after being kept at this temperature for some hours, 

 is slowly cooled. The prepared strip is now dipped into 

 the cell, which contains acetone, the other electrode being 



a platinum wire. If the two electrodes are now connected 

 to an electrometer, this instrument indicates a difference of 

 potential between the electrodes when the selenium surface 

 is illuminated. The selenium becomes positively charged 

 with respect to the acetone. The difference of potential 

 varies as the square root of the intensity of illumination. 

 Photo-electric cells of this character are extraordinarily 

 sensitive : by their means Minchin was able to detect 

 the effect of the light from planets and stars. 



SELENIUM BRIDGES.— An apparatus in which the 

 change in the electric resistance of selenium when illumin- 

 ated is studied may conveniently be termed a " selenium 

 bridge " to distinguish it from photo-electric cells with a 

 selenium electrode. Selenium is a verj' poor conductor of 

 electricity, but becomes more conductive when heated for 

 some time at 200° C, and then slowly cooled. In this 

 condition its electric resistance decreases under the influence 

 of Ught. The effect of light is practically instantaneous, 

 but the recovery on its withdrawal is gradual. In order 

 to render the fall of the resistance of selenium on illumin- 

 ation easily demonstrable, it should be arranged that the 

 initial resistance of the bridge is so low as to allow of its 

 easy measurement. This is readily managed b}' coating a 

 cylinder of mica with selenium, and winding a pair of 

 copper wires on it in a double spiral, so that the wires are 

 insulated from each other by the selenium. The resistance 

 between the wires should now be measured, and, if all is 

 right, this should be several megohms. The whole is now 

 heated and annealed, when the resistance in the dark \vill 

 be found to have fallen to a value easily measured. On 

 exposing such a bridge to bright sunlight, its resistance 

 may fall as much as fifty per cent. Bridges made on this 

 plan, and convenientlv mounted for experimentation, can 

 be procured from instrument-makers, who catalogue them 

 as " selenium cells." 



SELENIUM CRYST.\LS. — Most experimenters who 

 have worked at the curious photo-electric properties of 

 selenium have used the material in the state into which 

 it is brought bv slow annealing. The mass is then probably 

 a congeries of interlacing crystals. Recently single crystals 

 of selenium have been studied by Brown and Sieg {Physical 

 Review, ,\ugust, 1914 ; Philosophical Magazitte, October, 

 1914). By emplojang a needle-shaped crj'stal, it was first 

 shown that, when silver electrodes pressing on the crystal 

 were used, the resistance of the whole lay chiefly in the 

 ci-ystal, and not at the contacts. Next, it was proved that, 

 when the resistance decreases on illumination, the decrease 

 is due to a change of resistance in the crystal itself, and is 

 not limited to the contacts. So long as any light fell on 

 any part of the crvstal there was a marked fall of resistance, 

 even though the illuminated portion of the crystal was not 

 in the direct line of flow of the current. This transmission 

 of the effect of light to a distance was well shown in a remark- 

 able experiment. A long, thin crystal was held between 

 electrodes at one end, and a narrow beam of light was 

 allowed to play on the crystal at various points in its 

 length, while the resistance across the end between the 

 electrodes was measured. It was found that the fall in 

 resistance was as great when the light struck the crystal 

 at the end remote from the electrodes as when it fell close 

 to them. We thus have the amazing result that the action 

 of the light may be transmitted laterally to a greater dis- 

 tance than ten millimetres. The authors are incUned to 

 the view that, when light acts on a crystal, it operates 

 some mechanism which controls by secondary action the 

 conductivity of the whole crystal. 



SALTS COLOURED BY KATHODE RAYS.— An 



interesting paper on this subject was read before the 

 Physics Section of the British Association at the .Australian 

 meeting by Professor E. Goldstein, who discovered the 

 effect, and has done most of the work on the subject. 

 The paper is reported in Nature of December 31st, 1914. 

 If kathode rays fall on certain salts, such as common salt, 



