106 SUMMARY OF CUBRENT RESEABCHES RELATING TO 



ordinary development, and then, instead of fixing, the silver image i& 

 dissolved ont and the remaining silver salt reduced to the metallic state. 

 But if the work of the photographer himself is simple, it is because of 

 the complex character of the prepared plate ; and presumably it is the 

 difficulties of manufacture that have led to the delay in putting the 

 prepared plates on the market. The plates are made by selecting starch 

 granules of from 15 to 20 thousandths of a millimetre in diameter,, 

 staining quantities of them red, green, and violet respectively, drying 

 them, mixing them so that neither colour predominates, but that the 

 whole presents a neutral grey tint, and spreading the mixture on glass 

 one layer thick. The interstices are filled in with a fine black powder,, 

 and the layer is fixed and protected by a coat of varnish. On this is 

 put a film of suitably colour-sensitised emulsion. Exposure is given 

 through the glass, and the subsequent treatment of the plate is as 

 described above. The dyed starch granules form an irregularly grained 

 three-colour screen, which serves the double purpose of taking and 

 viewing. 



It is easy to describe such a process, but besides the obvious me- 

 chanical difficulty of preparing the plates, there must be many com- 

 promises made before the result can be passably satisfactory. The best 

 three colours for the exposure are not the best three for viewing the 

 picture, but in this case they have to be the same. If the stained 

 starch granules are mixed to the most neutral tint possible, it appears 

 that a perfectly orthochromatised sensitive film would be necessary. 

 The imperfections of the film in this matter must be neutralised as far 

 as possible. Indeed, the difficulties of which the photographer is 

 relieved have to be overcome by the manufacturer, and in this particular 

 case they are so many and complex that if it had not been stated that 

 results have been obtained in the manner described, we might very well 

 doubt the possibility of it. 



Photomicrography and Photomicrometry.* — J. Thompson employed 

 a fixed magnifying power (say lOOOdiams.) for photographing the object 

 to be measured. This is obtained by using an oil immersion T V with a 

 certain eye-piece, a fixed tube length, and screen distance. A sheet of 

 paper is ruled in squares. This is photographed by an ordinary camera, 

 and reduced until the squares measure 1 mm. on the negative. This. 

 negative is printed on the same positive as the photomicrograph ; a 

 direct measurement can therefore be made, because each mm. represents 

 a micron magnified 1000 times. Other fixed magnifying powers are 

 treated in a similar manner. 



Mathet, L. — Sur la reproduction desobjetsdifficiles par la photomicrograpbie. (A 

 series of articles on the photomicrography of difficult objects.) 



Bev. Sci. Photographiques, i. (1904) pp. 18-22, 48-53, 117-22, 



176-80, 231-4 (23 figs.). 



(5) Microscopical Optics and Manipulation. 



Aperture Table. — It will be noticed that the limii-fer resolving power 

 for white light in the aperture table, printed npon the fly-leaf of this 



* Proc. Scot. Micr. Soc, iv. (1903-4) p. 44 (pis. iii.-vi.). 



