DEFECTS IN NEGATIVES. 131 



The camera is to be examined in the same way. 

 Entrance of light need not be apprehended, if it 

 and the microscope tube are lined with black velvet, 

 and when in position covered with, a dark cloth. 

 The substitution of the carrier for the focussing 

 screen should take place underneath this or another 

 cloth, the rays from the lamp being cut off from 

 the microscope by a blackened card placed between 

 them, before the drawing up of the slide and its re- 

 placement. 



Actinic light must be guarded against after re- 

 moval from the camera, and during the subsequent 

 operations ; the slightest glimpse will produce a fog 

 on the negative. Fortunately this may be removed by 

 immersing the negative in a solution of bichromate of 

 potash, care must be taken that the clearing off is 

 not carried too far else weakness may be produced. 

 Another kind of fog may occur when using wet 

 plates, owing to alkalinity of the bath. A similar con- 

 dition of the oxalate of potash solution produces a 

 whitish deposit on the film of dry plates, it is there- 

 fore advisable to add a little sulphuric acid to this 

 solution after its complete neutralisation. 



Pin-holes are due to dust on the plate. A soft large 

 camel's hair brush specially kept for the purpose 

 should always be passed over dry plates before plac- 

 ing them in the dark slide. Black markings are 

 generally caused by the plate coming in contact with 

 the fingers, or some unclean surface, and insufficient 

 drainage before placing it in the carrier. 



Frilling is very liable to occur in hot weather with 



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