January, 1915. 



KNOWLEDGE. 



27 



PHOTOGRAPHY. 



Bv Edgar Senior. 



DEVELOP.MENT WITH FERROUS OXALATE 

 {continued from Volume XXXVII. page 436). — We liave 

 Ijefore us as we write some test negatives made in a sensi- 

 tometer and developed with ferrous o.xalatc, and these 

 show both good gradation and density, wliile the deposit 

 is of good neutral tint, the formulae for the two solutions, 

 and the strength of the developer employed, being as 

 follows : — 



No. 1. 

 Formula for Iron Solution. 

 Ferrous sulphate ... ... ... 1 ounce 



Water ... ... ... ... 3 ounces 



No. 2. 

 Potassium oxalate (neutral) ... 1 ounce 

 Water ... ... ... ... 3 ounces 



Both solutions should be made with liot water, and the 

 former shghtly acidified with sulphuric acid, and the latter 

 with oxahc. For development, four parts of number tvvo 

 were taken and one part of number one added. The mixed 

 solutions were then poured over the exposed plate ; 

 when development was considered to have proceeded far 

 enough, the film was rinsed ; the plate was then placed in 

 a saturated solution of common alum for about five minutes, 

 and then well washed and fixed. If the alum bath is 

 omitted a white deposit of o.xalate of lime will be seen in the 

 film. Various modifications of the method tif using ferrous 

 oxalate were also employed, and Dr. Liesegang recommended 

 that in order to produce soft negatives from subjects 

 having strong contrasts, the plate should be soaked for 

 about two minutes in a plain solution of iron sulphate, 

 to which was added about two or three drops of the dilute 

 " hypo " solution to each ounce of the iron solution. At 

 the expiration of a couple of minutes the sulphate solution 

 was poured off, and one of potassium oxalate applied, when 

 the image quickly appeared and rapidly gained the required 

 density. When, however, the densitj' was not sufficient, 

 a little of the iron sulphate was added to the oxalate, 

 and the mixture again appHed to the plate. In cases where 

 great brilhancj^ was the desideratum, the reverse method was 

 adopted, the plate being first soaked in the oxalate solution 

 for 1:wo or three minutes a little '' hypo " being added to 

 the oxalate and at the end of this time a full-strength 

 oxalate developer was apphed to the plate. By such means 

 as these, as well as alterations in the proportion of ferrous 

 sulphate used, variation in the type of negatives obtained 

 could be made. When a still stronger form of ferrous 

 oxalate is required, it may be prepared by the addition of 

 soUd ferrous sulphate to a saturated solution of potassium 

 oxalate. It is, however, in the development of bromide 

 paper that ferrous oxalate is Hkely to find its most useful 

 application now ; and, from considerable experience, the 

 wTiter can thoroughly recommend the following formulae 

 for general use in this direction : — 



For use six parts of No. 1 are taken and one part of No. 2 

 added. The mixed developer is then poured over the exposed 

 bromide paper, when the image will develop graduaUy 

 and of a good colour, ^\^len the development is almost 

 completed the solution should be poured oft into the measure, 

 and the moment the desired result is obtsiined the print 

 must be flooded with the following clearing solution : — 



Clearing Solution. 



Sulphuric acid i ounce 



Water ... ... ... ■■• 80 ounces 



or 



Acetic acid jounce 



Water 80 ounces 



Either of these solutions should be allowed to remain on 

 the paper for one minute, when a fresh one is applied, 

 wliich is allowed to act for a similar time, and, finally, a 

 third one is applied for the same time. The print is then well 

 rinsed and fixed in the following fixing bath : — 

 Fixing Solution. 



"Hypo" 3 ounces 



Water 20 



The time of immersion in the fixing bath should not be 

 less than fifteen minutes to secure thorough fi.xing. Prints 

 developed \\ith ferrous oxalate have very clean whites and 

 rich velvety blacks unless the exposure has been excessive, 

 when a brownish tint may be imparted to the latter. In 

 short, the only drawback to the use of this developer seems 

 to be its slowness when compared with amidol, and the 

 necessity for using three clearing baths. 



PHYSICS. 



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



DETAILS OF CRYSTALLINE STRUCTURE.— In 

 1912 Friedrich, Knipping, and Laue found that a narrow 

 beam of A--rays sent through a slab of crystal on to a photo- 

 grapliic plate produced a geometrical pattern of spots 

 instead of only one, as would have occurred if the rays had 

 been incapable of interference and reflection. The spots 

 can be accounted for by regarding the rays as having a wave- 

 length, and either treating^he crystal as a three-dimensional 

 diffraction grating, or as being capable of producing re- 

 flection from planes rich in atoms. The latter point of 

 view was advocated by W. L. Bragg, who has done much to 

 apply the new discover^' to the investigation of the details 

 of crystal structure. The simplest case yet studied is that 

 of naturally crystalline copper, which was shown by W. L. 

 Bragg {Phil. Mag., September, 1914) to consist of atoms 

 arranged as follows : At each corner of a cube whose edges 

 are 3^6. 10-" centimetres in length there is an atom, and 

 also one at the centre of each of the square faces. The whole 

 of the crystal is built up of a framework of atoms, all on 

 the same' plan, so that all these fourteen atoms will also 

 figure as members of adjoining cubes. Such an arrange- 

 ment is called " a face-centred cubic lattice." 



A crj'stal is capable of reflecting x-Ta.ys when these strike 

 the crystal at certain angles. Suppose a cubic section of 

 such a cn,-stalline scheme as the foregoing to be before the 

 eyes ; it will have five atoms on the top and bottom faces 

 of the cube, and four on the plane midway between the 

 top and bottom, so that planes rich in atoms run parallel 

 to the faces of the cube. It is also seen that all of these 

 planes have the same number of atoms per unit area. 

 Now, if a beam of ^r-rays fall on the top face of an assemblage 

 of a large number of atoms thus spaced, a reflected wave 

 starts from the top layer. The waves are not wholly 

 reflected ; thev travel on and strike succeeding layers, 

 which in their fairn send out reflected waves. If the wave- 

 length \, the distance apart of the layers /, and the angle 

 between the face of the crystal and the ;i;-rays or glancing 

 angle S be so related that each set of reflected waves loses 

 a whole number of wave-lengths on the set reflected from 

 the layer above, all these reflected waves will conspire 

 together and the crystal will act as a reflector. This is so 

 when 



n\=2l sin «„, 

 where n is the number of wave-lengths lost by the suc- 

 cessive sets of reflected waves, while e^ is the particular 

 glancing angle for this case. So that « corresponds to the 

 order of the spectrum in an ordinary grating. 



The wave-length of the x-Ta.ys used was -573. 10-"* 

 centimetres from a tube with an anticathode of palladium. 



