president's address. 245 



ask your indulgence for a minute or two while I make a few 

 preliminary remarks. 



Speaking briefly, there are two kinds of emulsion used for 

 coating photographic plates, the fast and the slow. Each has its 

 own characteristics. The fast has the property of picking up 

 and recording every little variation or " gradation" of light, as it 

 is called, never failing to make such differences apparent. The 

 consequent result of this property is that a photograph taken by 

 such an emulsion would not fail to show the fog surrounding the 

 dots to perfection. With the slow type it is just the contrary. 

 Its power of differentiation, or of dealing with very small 

 gradations of light coming from an object, is sensibly less than is 

 the case with the fast plate, so, in consequence, it would certainly 

 not show the .fuzzy haze nearly so well as the fast emulsion. 

 At once then you will say, Why not use such a plate ? Un- 

 fortunately it must be at once stated that the very fact of the 

 slowness of the sensitised film makes it impossible to do so, because 

 the length of exposure would be prohibitive. But the photo- 

 grapher is not beaten yet ; and I will now proceed to explain how 

 he can assist the photomicrographer by the employment of his 

 knowledge of the subject. 



This is the method. The first negative being taken on a rapid 

 plate, say at some thousands or more diameters, is developed pre- 

 ferably with hydrokinone to obtain as much contrast as possible. 

 If it is a good one, showing the dots or other secondary markings 

 sharply focussed, it is left to dry. When examined it will 

 be seen to show the veil of which we are complaining, and 

 perhaps such is well seen around the dots, so much so that 

 it gives the appearance of their being immersed in a bath 

 of fog. Perhaps the print may show this defect more 

 definitely than the negative itself. A fast plate is then 

 placed in contact (such a one as the " Flashlight " of the 

 Imperial Company), and the printing frame is waved once before 

 a sixteen-candle electric lamp, or some other powerful illuminant 

 placed about two feet away. This is developed as if it were a 

 negative, although, of course, it is a positive — by which is meant it 

 is developed by time and not for the purpose so much of being 

 used as an ordinary positive would be. It simply means that it 

 is a very well exposed and developed positive and not a very thin 

 and transparent one. The dots appear very plainly and sharply 



