June, 1913. 



KNOWLEDGE. 



223 



taking a solution of silver nitrate and one of hypo, when, 

 on adding them together in such a manner that the former 

 is present in excess, a white precipitate, which rapidly turns 

 brown and finally black, is the result, the reaction being 

 expressed by the following equations : — 



2AgNO a + Na 2 SaO» = Ag a SaO a + 2NaNO :i . 

 Then 



AgaSaO, + HaO = Ag a S'+ H2SO4. 

 By the addition of hypo in excess, however, the white pre- 

 cipitate first formed is readily dissolved and a perfectly clear 

 solution results. Therefore care should be taken to have the 

 fixing bath sufficiently strong and to allow the plates or prints 

 to remain in long enough to ensure perfect fixation. In 

 other words, to ensure the formation of the double thiosul- 

 phate of silver and sodium, which is highly soluble, a reaction 

 expressed by the equation : — 



2AgBr + 3Na 2 SaO :! = Ag s Na 4 (Sa0 8 ) s + 2NaBr. 

 ACID FIXING 

 BATHS.— The use 

 of an acid fixing 

 bath has certain 

 advantages over that 

 of a plain solution 

 of hypo in water, 

 inasmuch as the 

 bath keeps clean 

 for a much longer 

 time and the plates 

 are quite free from 

 any deposit or stain 

 in the film. Their 

 preparation requires 

 care to avoid any 

 precipitate of 

 sulphur taking place 

 due to decomposi- 

 tion of the hypo by 

 the acid. A very 

 good formula for ■»#■* — .•■ .----• 

 such a bath, " origin- 

 ally given by Mr. p^ j5 

 Sandell for fixing 



his double- and FlGl!RK 2i4 " Photographic test of 



triple-coated 

 plates," is the following : — 



Hypo ... ... ■•■ ■•• ••• 7 ounces 



Soda Sulphite 1 ounce 



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



Strong Sulphuric Acid 1 drachm 



After the hypo and soda sulphite are dissolved the 

 sulphuric acid is added in drops at a time, stirring all the while. 

 Such a fixing bath keeps quite a long time and may be 

 diluted or not as thought desirable. Another bath, containing 

 meta-bisulphite of potassium, is as follows : — 



5 ounces 

 20* " 



Hypo 



Potassium meta-bisulphite 



Water 



An acid fixing bath, containing common alum, can be prepared 

 by taking certain precautions in mixing. The following will be 

 found a very good formula : — 



Hypo ... ... ... ... ••• 4 ounces 



Soda sulphite 1 ounce 



Common alum ... ... ••• ••• i „ 



Tartaric acid 60 grains 



Water ... ... ... ••• ••• 20 ounces 



The ingredients must be dissolved in the order given, as, 

 supposing the soda sulphite were to be added last, it would be 

 found that the alum and tartaric acid would react with the 

 hypo and a precipitate of sulphur would result. 



A good plan is to dissolve the hypo and sulphite in a part of 

 the water and the alum and tartaric acid in the remainder. 

 The alum is then added to the solution of hypo and soda 



sulphite and finally the tartaric acid solution is poured slowly 

 into it, stirring all the while. A precipitate which first forms 

 will be found to dissolve up completely, and the acid solution 

 will remain quite clear. This fixing bath possesses consider- 

 able hardening properties when freshly made, which, however, 

 decrease rapidly by keeping. 



TESTING DARK - ROOM LIGHT FILTERS.— One 



frequently sees an otherwise good negative completely spoilt 

 through undue exposure of the plate during manipulation to 

 the light from the dark-room lamp. If we remember that 

 there is probably no such thing as an absolutely safe light — it 

 simply being a question of the length of time a plate may be 

 exposed without showing any effect due to this cause — it is 

 evidently advisable that some kind of test should be made to 

 ascertain it. Of course, the spectrum test is valuable in 

 showing the region of the light transmitted by the material 

 used as a filter, but then the general sensitiveness, as well as 



the spectrum sensi- 

 tiveness of the 

 emulsion, has to be 

 taken into consider- 

 ation also. And it 

 is found that a light 

 which is perfectly 

 safe to employ with 

 a plate of low sensi- 

 tiveness would not 

 be so for one of the 

 opposite nature. In 

 fact, a plate may be 

 orthochromatic, but 

 its general sensitive- 

 ness so low that it 

 may be developed in 

 a yellow light, with- 

 out any bad result 

 following from so 

 doing. The question 

 naturally arises then 

 as to what test 

 should be applied to 



ascertain the safetv 

 the safety of dark room light filters. of otherwise of the 



medium in use for 

 the purpose of filtering the light through. The one employed by 

 the writer is due to Sir William Abney, and consists in exposing 

 a plate behind several different kinds of coloured glass, paper, 

 or fabrics to the light used as the source of illumination for 

 periods varying with the nature of the emulsion on the plate. 

 A test of this kind is shown in Figure 234, the material used 

 being (a) canary medium, (6) cherry fabric, (c) orange paper. 

 These were fastened together and placed upon a piece of clear 

 glass in a printing frame, and a special rapid gelatine plate, 

 225 H and D, laid upon them. The whole was then exposed 

 to the light from a sixteen-candle power incandescent electric 

 lamp for a period of two minutes at a distance of two feet 

 from the light. The plate was then developed in darkness, the 

 time allowed being five minutes. Examination showed that 

 practically no action had taken place upon that part of the 

 plate under the orange paper, while the cherry fabric had 

 allowed sufficient light to pass to cause considerable action on 

 the surface beneath it, while that part of the film protected by 

 the canary medium was, as might be expected, practically 

 black. Instead of fixing the image, the film was simply 

 washed and then placed in a solution of bichromate of 

 potash, made acid with sulphuric acid. This quickly dissolved 

 out the reduced silver, and after a thorough washing in 

 running water the plate was brought into the light and 

 redeveloped with amidol. By adopting this method the 

 necessity of making a transparency in the usual manner is 

 done away with and we at once get the relative effects pro- 

 duced shown in their correct aspect of light and shade, and 

 the test furnishes the information that one thickness of orange 

 paper employed in front of a sixteen-candle power electric 

 lamp would practically afford such safety that a gelatine plate 



C 



