SCBEENS PLATES EXPOSUEE 221 



An alternative method of preventing halation is provided 

 in the Isolar plate. In this a red-coloured film is interposed 

 between the emulsion and the surface of the plate, so that no 

 actinic light passes beyond the lower side of the emulsion-film. 

 The red colour has to be discharged during the process of 

 development, and the makers of the plates give full instructions 

 how to do this. While it would appear in theory that this 

 method is superior to any method of ordinary backing, 

 yet in practice it sometimes happens that it is difficult to 

 discharge the colour thoroughly. 



Fig. 69 shows photo-micrographs of a slow backed plate (a) 

 and a fast unbacked plate (b). The magnification is 240 

 diameters, and it shows quite clearly the difference between 

 the size of the grain in a fast and a slow plate. The object 

 photographed is one of the lines in an Abbe test-plate, which 

 visually appear in the negative to be perfectly defined. The 

 dark and light portions in the illustration show that even 

 what appears to be a well-defined line is not actually perfectly 

 sharp ; but in the case of the slow plate the result is much 

 better than in the fast one. The difference in the grain is at 

 once obvious ; but in the case of the fast plate this was also 

 unbacked, so that the spreading action of the light as well as 

 the larger grain is shown up. 



In actual practice, although it has been recognised that the 

 results to be obtained on very slow plates are superior to those 

 on fast ones, yet for some hitherto unexplained reason it 

 generally happens that a plate of medium rapidity gives the 

 best result. In a paper recently read before the Eoyal Society 

 by Dr. Kenneth Mees, 1 on the resolving power of photographic 

 plates, it was shown that for rendering of fine detail, more 

 particularly in the applications of photography to spectroscopy, 

 a plate of medium rapidity might be expected to be the most 

 suitable. This result is dependent not entirely on fineness of 

 grain, but on light-diffraction or reflection within the film and 

 between the particles of silver themselves in the emulsion. 

 It is recommended that those who purpose doing work of a 

 highly critical nature especially with objects in which there 

 is fine recurring structure should refer to, and carefully read, 

 this paper. 



1 British Journal of Photography, December 24, 1909. 



