PHOTOGKAPHIC PKOCESSES 247 



will depend upon the light and density of the negative. But, 

 in general, it may he taken that it will be approximately twice 

 as long as for a dry lantern-plate of average speed. A full 

 exposure is advisable, as intensification of an over-exposed 

 plate is quite practicable. 



Exposure having been effected, the plate is removed 

 from the slide in the dark-room, and development carried out. 

 A suitable developing-solution is as follows : 



Ferrous sulphate . 1 gramme. 



Glacial acetic acid . 0-5 c.c. 



Alcohol ... 2 c.c. 



Water . . 100 c.c. 



The solution should be kept for a few days before use. It 

 tends to improve with age, but should be discarded when 

 turbidity becomes evident. 



About 10 c.c. of this solution is poured on to the centre 

 of the plate, the latter being held between the thumb and 

 finger, and the developer caused to flow from corner to corner. 

 It is important that it should flow over the plate as quickly 

 and evenly as possible, since the image comes up almost at once. 

 Sufficient solution should be used to secure this, but not 

 enough to cause any of the developer to flow over the sides. 

 The plate should be slowly rocked from side to side until 

 development is complete, when it should be gently rinsed 

 under the tap and fixed in a solution of hyposulphite of 

 soda of the usual strength. The image will appear much 

 sooner than in a gelatin plate, and will more quickly gain 

 the required density the whole operation being completed in 

 from one to two minutes. It is important to see that the 

 developer is not allowed to flow off the plate, as the action 

 depends to a considerable extent upon the free nitrate of 

 silver which remains in the film helping to bring up the image. 

 The loss of density in fixing is slight. Exposure and develop- 

 ment should be carried through as quickly as possible after 

 the preparation of the plate to avoid partial and uneven 

 drying. 



The advantage of the wet-plate now becomes evident. 

 It is possible to intensify a thin slide, or one from a flat negative, 

 and to make it almost, if not quite, as good as a slide from 



