424 Foot a. Strobell: A method of focussing in photoinicrography. XVIII, 4. 



Looking through tbe lens , focus with fine adjustment for tke 

 details wanted in tlie photograph, and when the right focus is se- 

 cured , let the microscope stand undisturbed with tbe lens still on 

 top of projection ocular, for fifteen minutes. Tben examine tbe 

 preparation again, and if tbe focus bas beld, reniove tbe lens 

 from top of ocular , draw down tbe bellows and expose tbe plate. 

 It is a waste of tinie to expose tbe plate unless tbe stability of the 

 focus is assured, for the slightest cbange of focus during exposure, 

 destroys the sharp outlines of the image, giving that blurred effect 

 so familiär in many photomicrographs. This slipping of the focus 

 we have found the most troublesome factor in photograpby, and 

 this danger must exist whether tbe vertical or horizontal carnera is 

 used , or with any method of focussing. * A worn or an imperfect 

 micrometer-screw is not the sole cause of this trouble, for we have 

 tested a new Zeiss microscope and we found the focus changed so 

 radically that after a half hour's wait, the centrosome (on whicb 

 the test was made) was completely lost sight of. We are inclined 

 to think that changing of the focus is due rather to variations of 

 temperature to whicb microscope may be subjected, for example, in 

 bringing it from a warm part of the laboratory and placing it close 

 to a window , though this would seem hardly adequate to account 

 for all the vagaries of a changing focus. Sometimes a wait of an 

 hour or more is needed to insure a stable focus , but fortunately 

 these are rare occasions , as a rule ten or fifteen minutes' test is 

 all that is needed. 



If the focus does not hold during the fifteen minutes, 

 remove the focussing lens, whicb for clearness of description we will 

 assume is — 7 D., examine tbe preparation through a — 5 D. and 

 tben through a — 9 D. This will show in which direction the focus 

 bas slipped. 2 If, for example, the focus through the — 5 D. looks 

 more like the focus originally selected, the focus bas slipped down, 

 and you must turn the micrometer screw up , and this should be 



them in a case systematically arranged. The hole at the base of the 

 bellows must be made large enough to allow the holder as well as the 

 eyepiece to slip in and out freely. 



■ ! ) This difficulty is not overcome by Short exposures, for the time 

 it takes to put in the plate is sufficient to mar the sharpness of a photo- 

 graph if the focus is not stable. 



-) We select lenses two points higher and lower than the — 7 D. 

 simply to accentuate this point. 



