122 THE USE OF THE MICROSCOPE 



4. Objective lenses too far apart. Objective lenses too close together. 



5^ Correction collar at too low a Correction collar at too high a 



number. number. 



6. Convex corrected lens behind Concave corrected lens behind ob- 



objective. jective. 



7. Drawtube too long. Drawtube too short. 



Since overcorrection of course cancels undercorrection, 

 any of the causes of overcorrection can be used to cancel 

 more or less any of the varieties of undercorrection, and 

 vice versa, especially when a green screen is also used. 



The following tests may be applied : 



Overcorrection Undercorrection 



1. Star test. Bright spot in silver Diffraction mist beyond focus of 



film (or nigrosin smear), or with objective. Sharp ring within 

 dark field. Faint, misty expan- focus, 

 sion within focus of objective. 

 Sharp ring beyond focus of ob- 

 jective. (Siedentopf.) 



2. Diatom test. Fine dots or lines Details focus within focus of coarser 



(Pleurosigma or Surirella) focus parts; i.e., the markings seem 

 beyond focus of coarser parts of below the valve, 

 valve, so that the microscope 

 has to be raised to focus the 

 dots. (Conrady.) 



On the Choice of Objectives. — For the different kinds 

 of microscopical work, either routine work or research, 

 the lowest single objectives have been commonly replaced 

 by the Greenough binocular, because of its stereoscopic 

 effect (and leading optical firms have lately added objec- 

 tives of 8 and 12 initial magnifications to the Greenough). 

 Hence the 16- (or 18-) millimeter objective of 10 (or 8) 

 magnification, which is needed as a finder, and which has 

 a higher aperture than any objective commonly used on 

 the Greenough, appears to be, as already stated, the lowest 

 objective usually needed on the standard microscope. 

 For the next higher objective, however, a 20-times (8 

 millimeters), or a 50 or 60 oil-immersion objective of 0.85 

 to 1.0 aperture, such as is now manufactured for dark 

 field, would, the writer thinks, be often preferable to a dry 

 40 objective. However, for routine work, a 40 achromatic 



