104 PRACTICAL PHOTO-MICROGRAPHY. 



the dark slide, are degraded and fogged, either the plates are 

 foggy by nature or the light of the operating room is unsafe. 

 If the protected parts of the plate are quite clear, we must look 

 to overexposure, or failing that to reflections inside the camera. 

 Black streaks across a negative mean stray light reaching the 

 plate, and the sharper are the outlines of the streak the nearer 

 to our plate must we look for the leakage. A hole in the dark 

 slide makes a much sharper streak than one of the same size in 

 the camera front. 



Uneven lighting is a very frequent cause of failure, but its 

 results on the negative are not difficult to recognize, except as 

 in next paragraph. 



There are many defects to which plates are liable, but we 

 cannot enter into details of such things here. Uneven coating 

 has often the semblance of uneven lighting ; careless packing 

 often produces details on our negatives not projected by our 

 objectives. 



Transparent spots of minute size, called "Pinholes" are 

 caused by dust adhering to the film, these spots have as a rule 

 no nucleus ; but dust in the emulsion usually produces spots 

 that have nuclei. Each plate must be dusted with a clean 

 camel hair brush before being put into the slide. 



