246 PKACTICAL PHOTO-MICKOGBAPHY 



before the plate is coated. A number of plates may be so pre- 

 pared and kept ready for use. The collodion may be purchased 

 ready iodised, and it is better to have some that has been kept 

 for about a month before it is taken into use ; in any case it 

 should have been kept long enough to have changed to a sherry 

 colour. The coating of the plate is not difficult when once the 

 knack has been acquired. Take one of the clean and rubber- 

 edged glasses, hold it by the thumb and finger at one corner, 

 and pour on it sufficient collodion to cover about one-third of 

 the plate. Gently incline it to each corner in succession, so 

 that the collodion is spread evenly over the surface. When 

 the last corner has been covered, tilt the plate and pour any 

 surplus collodion back into the bottle. While the plate is 

 draining, it should be rocked backwards and forwards in order 

 to avoid the formation of ridges as the film sets. The coating 

 operation can be performed in daylight, since up to this point it 

 is not sensitive to light ; but the next, the sensitizing process, 

 must be carried out in yellow or orange light in the dark-room. 

 The plate is now immersed in a silver-bath, made by dissolving 

 thirty grains of nitrate of silver in one ounce of distilled water. 

 The solution should be distinctly acid to litmus, and if it is not 

 so, must be made acid by the addition of nitric acid drop by 

 drop. The solution must be filtered before use. After coating 

 the plate with the collodion film it must be allowed to set for 

 from one to three minutes, according to the temperature, but not 

 sufficiently long for it to become dry in any part. The nitrate 

 of silver solution should be placed in a porcelain dipping-bath, 

 such as may be obtained from any dealer. The collodionised 

 plate having just set is immersed in the solution, and allowed 

 to rest there for about a minute. The plate should next 

 be raised and, if necessary, re-immersed until all appearance 

 of greasiness disappears. The plate may be allowed to remain 

 about a minute after this appearance has been reached, and it 

 will then be ready for exposure. The time it takes to sensitize 

 will usually vary from three to five minutes. The plate is put 

 into a dark-slide, which should have silver- wire corners fitted 

 to it to avoid contact with the film more than is absolutely 

 necessary, and the exposure made. 



As in dry-plate work, no definite instructions can be given 

 as to the length of time necessary for the exposure, for this 



