April, 1915. 



KNOWLEDGE. 



117 



with a short exposure than many of the alkaline developers. 

 This, however, appears doubtful, as it is believed that the 

 detail brought out is similar, whatever be the developer 

 employed. The advantage of eikonogen is that it prevents 

 that excessive contrast that is likely to be obtained at times 

 with "pyro" and still further with hydroquinone ; and the 

 great difficulty often experienced by those who have been 

 used to either pyro- or hydroquinone as a developer has 

 been that of not getting sufficient density in the image 

 with eikonogen. But the entire absence of fog allows of 

 the image being afterwards intensified to the required 

 extent without difficulty. Then eikonogen may be mixed 

 with either " pyro " or hydroquinone, or may be used after 

 these at any stage of development, so long as the alkali 

 employed as the accelerator is a carbonate. Thus, suppose 

 that, with pyro as the developer, there is the appearance 

 that the high lights are becoming too dense before the details 

 in the less illuminated portions of the subject are well out; 

 then these may be developed without any further perceptible 

 increase of density by washing off the "pyro" solution, 

 and applying one of eikonogen and carbonate of soda, the 

 quantities being regulated according to the stage that the 

 negative has reached. It should also be mentioned that 

 the solution of eikonogen should be kept as near as possible 

 at a temperature of 65° F., as it is very insoluble in cold 

 water, and will crystallise out, and therefore lose nearly 

 all its developing power. With this developer the quantity 

 of sulphite required appears to vary with the nature of the 

 subject and the plate used ; it must, however, be at least 

 equal to that of the eikonogen, and may often with advantage 

 be increased to twice, or even three times, that. Except 

 for very quick exposures, a small quantity of bromide- 

 should be used in the developer, as it appears to have the 

 effect of producing greater density in the negative. As, 

 however, restrainers act much more powerfully than with 

 pyro, only three or four drops of a ten-per-cent solution 

 should be added to each ounce of developer ; and the bro- 

 mide must be potassium, and not ammonium. Develop- 

 ment with eikonogen may also be conducted tentatively 

 by adding the alkali in small quantities at a time. A 

 saturated solution of potassium carbonate should be made, 

 which may be added in drops as development proceeds. 

 The developer may also be allowed to remain stationary 

 upon the plate without markings of any kind appearing, 

 provided that the solution is quite free from any deposit ; 

 and, if required, further development can be instantly 

 checked by immersion of the plate in a one-per-cent. 

 solution of either acetic or citric acid. Over-exposure 

 can, to an extent, be corrected by increasing the quantity 

 of sulphite in the developer, although eikonogen cannot be 

 said to allow of much latitude in this direction. Oi the 

 many formulae that have been given from time to time the 

 following will be found useful for portrail oi" landscape 

 work generally : — 



Portraits and I. ami'-' mi 



rains 



Eikonogen... ... ... ... ,5 



Soda sulphite 22 



Soda carbonate ... ... ... 16-5 



Water (distilled) ... ... 1 ounce 



For very quick exposures the following formula is to be 

 recommended : — 



For Very Short Exposures. 



Eikonogen 14 grains 



Soda sulphite ... ... ... 75 



Potassium carbonate ... ... 30 



Water (distilled) ... ... ... 1 ounce 



Then, again, eikonogen is especially suitable for use with 

 isochromatic plates, and, when mixed with hydroquinone, 

 yields results on these plates that arc excellent in ever] 

 way. The following formulae, which were originally 

 published by Carbutt, are for a developer of this nature 

 which the writer has used repeatedly with great success : — 



Carbutt's Formula. 



For Snapshots on Isochromatic Plates. 



Eikonogen... ... ... ... 2 grains 



Hydroquinone ... ... ... 1 grain 



Soda sulphite ... ... ... 12 grains 



Potassium carbonate ... ... 6 ,, 



Water (distilled) ... ... 1 ounce 



For Portraits on Isochromatic Platks. 

 Eikonogen... ... ... ... 1-6 grains 



Hydroquinone ... ... ... 0-8 grain 



Soda sulphite ... ... ... 10 grains 



Potassium carbonate ... ... 5 



Water (distilled) ... ... 1 ounce 



For Landscapes ox Isochromatic Plates. 

 Eikonogen... ... ... ... 1-9 grains 



Hydroquinone ... ... ... 0-95 grain 



Soda sulphite ... ... ... 11 grains 



Potassium carbonate ... ... 3-6 



Water (distilled) ... ... ... 1 ounce 



Care should be taken not to over-expose the plates, 

 when these mixed developers will be found to do everv- 

 thing required, if the development is carried far enough. 

 Sufficient has now been said as to the value of eikonogen 

 alone, as well as when mixed with hydroquinone ; and, 

 as the substance can be obtained at a cost less than that of 

 pyro at the present time in many cases, it is worth while 

 giving it a trial. 



PHYSICS. 



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



THE COOLIDGE AT-RAY TUBE.— This tube diners 

 very much from the usual form of .t-ray bulb, both in con- 

 struction and in the principles underlying its operation. 

 It is manufactured by the British Thomson-Houston 

 Company, which has communicated a detailed account of 

 Dr. Coolidge's invention to The Electrician (Januarv 15th, 

 1915). The usual concave cathode is replaced in the new 

 tube by a spiral of thin tungsten wire, which is heated to 

 incandescence by a small battery. This wire gives out the 

 necessarv stream oi corpuscles, which, when they strike 

 the anti-cathode, givi- rise to the i-ravs. Tin- anti-cathode 

 also serves as the anode, and is made of wrought tungsten. 

 When the tungsten wile is cold the tube is capable of with- 

 standing an electrical pressure oi one hundred thousand 

 volts without the insulation of the vacuum breaking down, 

 so perfect is the exhaustion. When, however, the wire is 

 hot, the tube becomes conducting in one direction, namely, 

 through the space from the tungsten plate to the spiral, 

 the current being carried entirely by the stream of corpuscles 

 issuing from the hot wire. It is claimed for the new tube 

 that its action is more easily controlled than that of the old 

 tube, anil in particular that the conditions ,,f working are 

 more readily reprodu 



VICKY SHORT-WAVED ULTRA-VIOLET LIGHT.— 



The wave-length oi the light in the middle of the visible 



i 'i ■> brum is about six thousand tenth metres, a tenth metre 



being 10 -, ° metres. Stokes fust studied the ultra-violet 



region Ol the spectrum, and carried his work as far as 



length 1800 oi so The region lying between 4000 

 and 2000 may be called the ordinary ultra-violet region 

 Tins radiation is remarkable for us photographic and 

 fluorescent effects. Schumann many years ago earned the 

 knowledge ol the ultra-violet up to the estimated wave- 

 length 1000 The whole work had to be executed in a 

 rOSCOpe which could be evacuated, as air is opaque 



to these shorl wavi Special photographic plates had 

 to be used, and the prisms and lenses construct) 

 fluorite. By the use oi a concave grating in a vacuum, 

 Lyman has recently pushed the limit ol the spectrum up 

 to nine hundred tenth metres, and has hopes oi still further 

 invading the unexplored region lying between the shortest 

 known ultra-violet waves and the longest waves oi the 

 X-rayS. These have a wave-length ot the order of one 

 tenth metre, while shorter still are the y-waves given out 



by radio-active bodies. 



