204 THE APPLICATION OF PHOTOGKAPHY TO THE 



and it is, of course, much more accurate, as well as safer, to 

 obtain the desired effect by a large change than by a very minute 

 one. 



The time of exposure will depend on a great variety of condi- 

 tions : — the character of the light used, the density of the object, 

 and the amount of amplification. For instance, supposing we are 

 using a four-inch lens at a certain distance, it may take fifteen 

 secofids to obtain a satisfactory impression, but if we used a 

 one-eighth on the same object and at the same distance, it, in all 

 probability, would require forty inifwtes. 



The light which I have found most convenient is that of a 

 duplex paraffin oil lamp, but sun light, diffused day light, the lime 

 light, or the magnesium light, may be used. 



And, lastly, we have to consider the chemical operation for 

 developing the image. The active agent in this process is Pyro- 

 gallic acid in presence of Ammonia; but experience shows that 

 its effect is greatly increased by employing in combination with it 

 other re-agents, whose action is of a restraining character. 



The formulae I use are : — ■ 



Citric Acid ... 60 grs. 

 Water 8 oz. 



A. — Pyrogallic Acid ... i oz. 

 Sulphate of Soda. 2 oz. 



For use mix i part A. to 15 parts water. 



B. — Liq.Am. Fortiss. '880, i oz. I Sulphate Soda ... 2 oz. 

 Bromide Amnion... \ oz. | Water ... ... 8 oz. 



For use mix i part B. to 15 parts water. 



For a \ plate (3J by 4J) take about 5 or 6 drs. of the dilute 

 B. solution, and float it evenly over the surface of the plate, laid 

 on its back in a shallow tray. Remove all air-bubbles from the 

 surface with the finger or a soft brush, and when the surface is 

 fully wetted, add an equal quantity of the dilute solution A. If 

 the time of exposure has been sufficient, the image will begin 

 to make its appearance in about two minutes; but the full 

 development may take from ten minutes to a quarter of an hour. 

 The process may be continued so long as the parts of the plates 

 that have been protected from the action of the light remain clear. 

 It is better to carry on the action too long than to arrest it too 

 early ; the impression loses a good deal of force in the after- 



