142 



DEFECTS IN NEGATIVES. 



allow a trace of ammonia to be present where the 

 wet plate process is employed. 



Among errors of manipulation may be classed 

 those in which the plate is fogged by the entrance 

 of actinic light. This is most liable to occur when 

 working with very sensitive dry plates. An enu- 

 meration of the various means by which it may gain 

 access to them and the precautions to be taken for 

 its prevention, although incidentally mentioned in 

 other places, will be repeated here. 



The dark room may allow light to enter by the 

 door, or the ruby glass window. The former may 

 be remedied by hanging a curtain both outside and 

 inside, the latter by the addition of another sheet of 

 ruby glass or non-actinic paper. The lamp may be 

 defective, and is almost certain to be so unless 

 covered by the tin shade, with a box beneath it, as 

 before described. 



The box in which the plates are stored may not be 

 light tight ; keeping it enveloped in a rug or dark 

 cloth is the best preventative, care being taken that 

 the dark room door is properly fastened before the 

 box is opened. When a plate has been removed the 

 box should be shut and covered up at once, other- 

 wise it might easily be forgotten. 



Prolonged exposure of the plate to the light, even 

 when filtered through two thicknesses of double 

 flashed ruby glass, will produce a fog. The inter- 

 val therefore during its transmission from the box 

 to the plate carrier should be as short as possible. 



Every point of the plate carrier must be examined, 



