52 PRACTICAL PHOTO-MICKOGKAPHY. 



including the supplementary tapereb! bellows seen in front, is 

 30 inches without using the rack and pinion ; an extra length 

 of 10 inches can be added at will. The writer has an, attach- 

 ment to the apparatus, Fig. 16, by which he can increase 

 his stretch of camera to 7 feet 6 inches, and his plate to 10x8 

 inches, but this is hardly ever used and never has been used 

 with real success. A stretch of 40 inches from micro-tube to 

 plate is perhaps the limit of utility, for very few object glasses 

 will stand more than that stretch in the writer's experience, 

 especially with oculars. 



The largest useful size of plate is about 7x5 inches, or what 

 is called in England " half-plate," 6x4f inches. The camera 

 should take a plate of one of these sizes, but should have " car- 

 riers" or "kits" fitting the " dark slide" to take 5x4 or 

 4Jx3J plates, the latter being technically called "quarter- 

 plates." Square plates are probably better than oblong ones 

 as a rule, so that the carriers may be made for 4J inch square 

 plates, but plates of unusual sizes are not so readily obtained 

 as common sizes. 



One point regarding the dark slide is very important ; it 

 should on no account slide into its position in the camera by a 

 long groove, but should be so made as to slide only about an 

 inch, or better still it should fall into a groove at the lower side 

 and be held into position by a catch at the top. There is very 

 little danger of light fog in this region of our apparatus, 

 especially as the room should be darkened (see later); 

 and there is very great danger of moving some part of the 

 apparatus after all is focused if the operator has to exert any 

 force to shove the slide into position. The shutter of the dark 

 slide must also work very easily and sweetly for the same 

 reason. 



The camera has of course a " ground glass," which is used 

 for preliminary examination of the image upon the screen, and 

 in certain cases the ground glass \ifinely ground is all-sufficient. 

 In any case the ground surface may with advantage be oiled ; 

 but even then the surface is too coarse for focusing images 

 with very fine details. Many devices have been used and 

 recommended for producing a surface sufficiently fine yet with 



