DEFECTS JN NEGATIVES. 143 



and its accurate closure ascertained, before the plate 

 is consigned to it; and afterwards covered with a 

 dark cloth. 



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 bichro- 

 mate of potash, care must be taken that the clearing 

 off is not carried too far else weakness may be pro- 

 duced. Another kind of fog may occur when using 

 wet plates, owing to alkalinity of the bath. A similar 

 condition of the oxalate of potash solution, and too 

 much iron in the mixed developer produces a 

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

 fore advisable to add a little sulphuric acid to oxalate 

 of potash 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 pur- 

 pose should always be passed over dry plates before 

 placing them in the dark slide. Black markings are 



