94: PRACTICAL PHOTO-MICROGRAPHY. 



The details that first appear under any of our normal devel- 

 opers must be most carefully scrutinized, because upon their 

 appearance we shall to a great extent base our judgment of 

 exposure. If the first details are pale gray and are hurriedly 

 followed by others almost equally gray the plate is over-ex- 

 posed ; if the first details rapidly become black, while other 

 details are either pale gray or invisible, the plate is certainly 

 under-exposed. If the whole plate become very quickly gray 

 over-exposure is certain; if the whole plate become dense 

 black, or dense black and dark gray, a less degree of over-ex- 

 posure is the probable cause. 



Experience, and a certain amount of allowance for the 

 nature of the subject, are both necessary in judging of ex- 

 posure by observation of the appearances during development ; 

 but when experience has been gained, and a variety of sub- 

 jects photographed, the worker will be able very accurately 

 to judge where and how much he has erred if he has erred 

 in exposure. But two things are settled : An image 

 homogeneous in color all over is an over-exposed image ; an 

 image which is white in any part when development is com- 

 plete is an under-exposed image, unless we have a subject 

 requiring absolute blackness in our print, such, for instance, as 

 a "dark-ground" subject. If some part of our object be 

 nearly opaque or highly non-actinic in color, as many patho- 

 logical and physiological red stains, we have but two courses 

 open to us. 1st. To deliberately over-expose the high lights 

 or actinic colors to such an extent as will allow development 

 of the opaque or non-actinic parts. Yery often this method is 

 unexpectedly successful, the high lights not being so much 

 overexposed as might have been feared ; or 2nd. We must use 

 a color correct plate, cutting off, if necessary, the over-actinic 

 rays by suitable screens. (See chapter XYI.) 



Another matter resting chiefly on experience, and almost 

 impossible to treat usefully in a book, is the amount of devel- 

 opment required. In most cases when details are all " up," 

 not necessarily distinguishable but at least developed, the 

 density of the negative after fixation would not be sufficient 

 to yield good prints by any of our usual processes. Now in 



