138 PRACTICAL PHOTO-MICROGRAPHY. 



In enlarging, as in most photographic processes, the crux of 

 the whole affair is the exposure, and it is just as hopeless here 

 for us to attempt to give rules for all conditions as it is else- 

 where. All we can say is that cc&teris paribus exposure varies 

 directly as diameters of enlargement. That is to say : with a 

 given negative, given light, given lens, given diaphragm, and 

 given sensitive material, it will take twice as much exposure to 

 enlarge a quarter plate to 8-J x 6 as it will take to enlarge it to 

 6 J- x 4J. The best way is to make a trial exposure on a small 

 piece of the paper and at the distance to be used seriously 

 and to develop it, noting carefully our remarks as to the 

 appearance on development of under and over-exposed prints 

 under the appropriate headings. With an arrangement, as 

 shown in Fig. 32, and an average negative, using a 13-inch 

 rectilinear at full aperture () and Eastman's bromide paper, the 

 daylight being of average autumn quality, to enlarge three 

 diameters the writer exposes from five to six minutes with 

 ground glass diffuser, rather less with white reflector alone. 

 But his average photo-micrograph ic negative is a dense one 

 compared with a landscape negative, not to mention a portrait 

 one. If the lens has to be stopped down to prevent spherical 

 aberration, the exposure will be greatly increased, generally 

 double for each next smaller size of stop as sent out by opticians. 



A " table of enlargements " will be found in this book, and 

 with it the worker may easily reckon approximately the posi- 

 tion of lens and easel for any given operation. The camera 

 bearing the lens may at any rate be racked to the desired ex- 

 tent, and then the focusing may be accurately performed by 

 sliding the easel. The front motions of the camera may be 

 used to arrange the image suitably on the screen. Focusing, 

 if not done from behind with ground glass as suggested, may 

 , be done by viewing the projected image on a white sheet of 

 paper afterwards to be replaced by the sensitive paper or glass, 

 but in our experience the former method is vastly superior. 



Enlarging ~by optical lantern : This is perhaps the favorite 

 system among amateurs who are likely to possess an optical 

 lantern for its ordinary use. Some of the laws touching the 

 optics of this system must be noticed. 



