34 Cincinnati Society of Natural History. 



The duration of exposure depends on so many conditrons that 

 the requirements of each plate must be determined on its own 

 merits. Fewer mistakes in exposure will be made if before begin- 

 ning operations the negatives be carefully looked over and sorted, 

 putting together those which nearest approach each other in den- 

 sity, and which therefore will require approximately the same ex- 

 posure. Negatives which require special treatment should also be 

 put aside and handled together, as experience gained by dealing 

 with one may be of great service in operating with the next. It 

 frequently happens that a negative otherwise good may have a 

 very thin foreground or a faulty sky, which may be corrected liy 

 shading the thin portion during the exposure. For this purpose a 

 piece of opaque paper or thin board may be used, but it must be 

 kept in motion so as to prevent the appearance of shading lines in 

 the copy. Many negatives too thin to print may be made to yield 

 excellent positives on glass by shading them with ground glass or 

 tissue paper, and giving a scant exposure, followed by slow, care 

 ful development. 



The copying camera should be jjomted toward a clear sky, 

 or toward a part which is evenly covered with clouds. No in- 

 tervening trees nor buildings should appear on the ground glass of 

 the camera when it is examined with the negative removed from 

 the frame. If any such image can be seen, no matter how indis- 

 tinctly, it will appear as a dark spot on the finished positive, and 

 as the cause will not be suspected, it may result in the loss of 

 much time and many plates. 



For work at night, the negative may be lighted, by one or 

 more lamps with reflectors, but great care is required to secure an 

 even illumination. With the best of the artificial lights which are 

 ordinarily within reach, however, a much longer exposure will be 

 required than for daylight work. 



Until quite recently all the best lantern slides were made by 

 the wet-plate process, in fact there were no gelatine dry plates 

 manufactured on which a more than passable lantern slide could 

 be made. At present there are several makers who produce plates 

 on which it is easy to make lantern slides of excellent (juality, 

 which arc only with difficulty to be distinguished from the best 

 wet-plate work. The latter, however, maintains its position as the 

 standard, on account of its perfect purity in the lights, its trans- 



