106 



KNOWLEDGE. 



March, 1913. 



PHOTOGRAPHY. 



By Edgar Senior. 



REVERSED NEGATIVES.— For certain photographic 

 processes, such as single transfer, 

 carbon, and photo-mechanical 

 printing, negatives are required 

 in which the image is reversed 

 as regards right and left, in order 

 that the finished impression may 

 appear in its correct aspect. The 

 production of negatives of this 

 nature may either be accomplished 

 directly in the camera or indirectly 

 by stripping the film from its glass 

 support and turning it, or by 

 reproduction from ordinary nega- 

 tives. By the first method the 

 reversal is generally obtained 

 through the use of either a metallic 

 mirror placed at an angle of forty- 

 five degrees with respect to the 

 axis of the lens, or by means of 

 a right-angled prism, from either 

 of which the image is obtained by 

 reflection ; in the latter, from the 

 hypotenuse surface of the prism ; 

 and in the former, from the 

 silvered surface of the mirror. 

 As both of these pieces of appa- 

 ratus are expensive, and, especially 

 in the case of the mirror, easily 

 damaged, a plan very frequently 

 followed, and which, at the same 

 time, is capable of giving good 

 results, is to reverse the position 

 of the sensitive plate in the dark 

 slide. When working in this manner it is, of course, 

 necessary to turn the focusing screen, for the purpose of 

 making allowance for the difference in the image plane due to 

 the thickness of the glass. 



By the second method the film 

 itself is stripped from its glass 

 support. This may be accom- 

 plished in several ways. In the 

 case of collodion negatives a 

 coating of india-rubber dissolved 

 in benzole is applied to the plate, 

 and when this is dry (which only 

 requires a few minutes) a film of 

 stripping collodion is applied and 

 allowed to become dry ; the nega- 

 tive is then placed in a dish of 

 water containing acetic acid, when, 

 after several minutes immersion, 

 the film begins to loosen from 

 the glass, from which it finally 

 floats free, and may be turned and 

 floated on to another piece of 

 glass which has received a coating 

 of gelatine. We have also found 

 the " Lotus stripping films " made 

 by Messrs. Mawson & Swan very 

 satisfactory, in which case the 

 negative is permanently removed 

 from the glass and used as a film. 

 When gelatine is the medium in 

 which the image is formed its 

 removal from the glass plate is 

 not so easily accomplished, apart 

 from the expansion (which is often 

 unequal) which the gelatine under- 

 goes, and the consequent distortion 

 of the image when transferred to another plate. A method 

 was introduced years ago in which the gelatine plate was 

 placed in a solution of fluoride of soda and citric acid, and 



Figure 101. 

 Print from the original negative 



Figure 102. 

 Print from a reversed negative 



although this method was found useful, the expansion was 

 considerable and increased with the quantity of citric acid in 

 the solution. To counteract this the application of a film of 

 plain collodion was often resorted to. Owing to the evil 



results attending the operating 

 with wet films, methods of stripping 

 dry were introduced, the film being 

 first thoroughly hardened by 

 formalin in order to lessen its 

 adhesion to the glass plate. For 

 this purpose the following formulae 

 due to E. Valenta may be 

 employed : — 



Formalin ... 150 minims. 

 Water ... 7 ounces. 



The negative is allowed to soak 

 in this solution for ten minutes, 

 after which it is dried and coated 

 with enamel collodion. The collo- 

 dion film should be a thick one, 

 and, when set, is well washed, and 

 the negative cut through with a 

 sharp knife about one-eighth of an 

 inch from the edge of the plate all 

 round. After drying the film with 

 filter paper, the stripping is pro- 

 ceeded with, and, when accom- 

 plished, the negative is immersed 

 in the following bath : — 



Alcohol 2 ounces. 



Glycerine ... 2 ,, 



Water 35 



On removal from this solution the 



film is laid, collodion side down, 



upon a piece of glass which has 



been rubbed over with a little of 



the glycerine mixture, when a 



piece of filter paper is laid on and a roller squeegee gently 



applied, after which the plate is stood up and allowed to dry 



spontaneously. By the third method the reversed image is 



obtained by reproduction from an 

 ordinary negative. This may be 

 accomplished in several ways, one 

 of which is to place a dry gelatine 

 plate in contact with the negative 

 in a printing frame, and expose to 

 a strong source of light (sunlight) 

 for a sufficient length of time 

 to obtain a reversed action of 

 light. A better plan, perhaps, is 

 to employ the method adopted in 

 the case of the example shown in 

 Figure 101, which is due to Mr. 

 Bolas : — A gelatine dry plate was 

 soaked for three minutes in a 

 four per cent, solution of potassium 

 bichromate, after which it was 

 rinsed for a few minutes in a bath 

 consisting of equal volumes of 

 alcohol and water. The super- 

 fluous liquid having been removed 

 by means of filter paper, the 

 plate was allowed to dry in the 

 dark. When dry it was exposed 

 under the negative from which 

 the print in Figure 101 was made, 

 for a time that would be required 

 to obtain a carbon print from 

 the same. After removal from 

 the printing frame the plate was 

 washed until free from the bichro- 

 mate, and the image then developed 

 by means of pyro and soda. After 

 fixing and washing a negative in which the objects depicted 

 were in the reverse aspect of the original negative resulted, 

 and from which the print in Figure 102 was made. The 



