90 PRACTICAL PHOTO-MICROGRAPHY. 



1. Under-exposure; 2. Reflected light inside the apparatus ; 3. 

 Too much light, or rather, too much angle not necessarily 

 over-exposure (over-exposure, under certain conditions and to 

 a certain degree, causes also grey backgrounds; but in this 

 case, as we shall see later, the whole image is grey). 



If we are dealing with a colored object we are practically 

 compelled to disregard all circumstances except that of color. 

 Color upsets every calculation of exposures that human ingen- 

 uity can devise. Reds and yellows sometimes cause us to in- 

 crease our exposure a hundred fold, but there are reds which, 

 being bluish (as eosin, a favorite stain with many for certain ob- 

 jects), upset all our previous calculations. Violets are, of course, 

 as a rule, highly actinic, aud require very brief exposures, but 

 logwood, as an example, stains certain tissues to a violet so full 

 of red that again we may be completely at sea in our exposure. 

 The writer has daily experience of such puzzling conditions. 

 Yellows, in like manner, are in certain objects practically 

 almost chemical opacity, while in other cases their contrast 

 with the white ground is so small as to render great the diffi- 

 culty of differentiating between the yellow and pure white. 

 A red, a violet, and a yellow may each be either very easy or al- 

 most impossible to render by ordinary photography ; a mixture, 

 such as a double stain of violet and red, is very often, with- 

 out " color correct " or " orthochromatic " photography, a com- 

 plete impossibility. 



In view of conditions so common, yet so puzzling, we again 

 submit that any table of, or rule for, exposure would be out of 

 the question : what we may do, and propose to try to do, is to 

 give guides by which the reader, on developing his negative, 

 whatever the subject may have been, may be able to correct 

 at next trial any error he may have made in his first exposure. 



Appended to each of the illustrations of this book we have 

 noted the exposure given by us in producing the negative, but 

 even this attention on our part is only of minor value on 

 account of our inability to gauge the quantity and quality of 

 of the light actually reaching our sensitive plate. 



