ELEMENTARY PHOTO-MICROGRAPHY. 33 



power microscopic objective, as shown in fig. 7. 

 A Davis shutter, or small iris diaphragm, to " stop 

 down " will be helpful in controlling the depth 

 of focus, and getting sharper outlines than would 

 be possible with a larger aperture. 



Special lenses may now be obtained to be used 

 without eyepiece, which are very suitable for 

 photographing large specimens. 



Watson's " Holostigmatic " lens has an iris dia- 

 phragm and standard gauge thread to fit any 

 ordinary microscope, or it may be used in the 

 adapter at the end of camera, as just described. 

 It gives splendid definition, has a rapidity of f/6, 

 and a large flat and uniformly sharp field free from 

 distortion. The diaphragm opening, correspond- 

 ing to a circle of confusion of .oo3in. for various 

 magnifications, is stated to be as follows : 



Stop. Magnification. 



f/45 is sufficient for x 2 

 f/32 x 4 



f/22 ,, X 6 



f/i6 x 8 



f/II ,, X 12 



f/8 x 18 



f/6 x 24 



In most cases the best results will be obtained 

 by using the diaphragm opening given in this list, 

 but it may sometimes be advisable for a thick 

 object to use a smaller one to get the required 

 depth of focus. 



