APPENDIX 2 



NOTES ON THE PHOTOGRAPHIC TECHNIQ^UE 



Though photography has been very extensively employed as an integral part of observa- 

 tion at every stage, there is nothing specially remarkable about the photographic 

 technique for straightforward observations such as those reproduced in half-tone 

 throughout the book. For photomicrographs of haematoxylin and gentian violet pre- 

 parations and also for natural-size photographs, Ilford Special Rapid Panchromatic 

 plates with appropriate colour filters were used. For carmine mounts some of the later 

 photographs were taken on Thin Film Half-Tone plates, since these give greater contrast 

 especially with a red object. 



The methods used for the making of text-figures are perhaps less well known and the 

 following processes may be of interest: 



(a) Photographic basis for drawings. A camera lucida was not used for any of the black 

 and white diagrams, but all were drawn on the basis of a photograph which in many 

 cases is the photograph reproduced. The original photographs were all taken at standard 

 magnifications of 500, 1000 or 2000 diameters, according to circumstances and from 

 them drawings were made according to the following procedure. An enlarged print at 

 a higher magnification (2000, 3000 or 4000) is made on to matt-surface bromide paper. 

 This is then inked over in the usual way, after which the photographic image is bleached 

 (see below) and the drawing alone remains. A very suitable paper for this purpose was 

 found in Kodak WSM. 3.S which has a very convenient texture both for drawing and 

 for subsequent treatment, but any ordinary matt paper can be used. The advantage of 

 tliis method of obtaining diagrams is that far greater accuracy is achieved than with a 

 camera lucida and with much less effort to the obseri^er; a thing of importance where 

 nuclei of the complexity of those shown here are to be analysed. 



(b) Method of bleaching. Any normal photographic method can be used, but to avoid 

 the handhng of KCN, which is a dangerous reagent outside a chemical laboratory, the 

 following procedure may be recommended as having proved satisfactory. 



A bleaching solution is prepared immediately before use by adding sufficient of a 

 stock solution of 10% potassium ferricyanide to the normal stock solution of 20% hypo 

 to produce a deep yellow colour. (The exact strength is not critical, but it is consider- 

 ably stronger than that used for normal reduction.) The print to be bleached is then 

 soaked in water until uniformly hmp and is then immersed in the bleaching solution 

 until the image has disappeared. The diagram which remains is then rinsed in water, care 

 being taken not to rub the ink, which is otherwise Hable to come off or run, and is then 

 given a brief bath in acid hypo to prevent or to remove yellow stains. It is then washed in 

 running water for half an hour. To assist drying it may be gently blotted. When dry 

 it may be necessary to touch up the ink here and there and the diagram is then ready. 



(c) Duplication of drawings by means of a paper negative. If it is undesirable to use the 

 original drawing for subsequent purposes it is a very simple matter to obtain facsimiles 



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