PRACTICAL EXERCISES WITH THE MlCROSCOl'E 261 



slide i)lus cover. Or measure the cover by focusing from 1 lie 

 under to the upper surface with a dry objective, and 

 multiplying the reading of the micrometer milled head in 

 microns by \]4. Set the correction collar at this point. 

 Use the largest possible cone which can be employed with- 

 out glare. Try, on a fine, well-stained part of the object, 

 the effect of shifting the collar one or more grades each 

 way. Finally, get the collar correct to half a grade. Try 

 to demonstrate that the objective gives the optimum image 

 with a cover-glass of 0.17 millimeter, notwithstanding the 

 setting of the collar at the right points for the different 

 thicknesses. 



30. Different Cover-glass Thicknesses with Oil-immer- 

 sion Objectives. — Take an oil-immersion objective 60, of 

 1.3 aperture. Make an India-ink, silver, or nigrosin film, 

 covered with immersion oil under covers of 0.17, 0.10, and 

 0.0 millimeter thickness. Using the star test, find the best 

 tube length for these three thicknesses of cover. Do the 

 same with an oil-immersion objective 90, of 1.3 aperture. 

 The 60 objective will probably require about 1 centimeter, 

 and the 90 objective about IJ'^ centimeters of increase of 

 tube length for uncovered objects. (Coles.) 



31. Resolution of Diatoms. — It is not only the fineness 

 of the markings which prevents the easy resolution of 

 diatoms; but it is often lack of sufficient contrast in the 

 markings, also. For the most difficult of diatoms are 

 resolved more readily when mounted in realgar, so as to 

 give the maximum of contrast. Hence, to resolve the 

 markings fairly readily, mount the diatom in hyrax, not 

 in balsam or immersion oil, and use strong almost mono- 

 chromatic green illumination, by focusing a direct tungsten 

 ribbon through several superposed green screens, like Nos. 

 58, 61, or 64, of the Wratten series, the immersion condenser 

 being well corrected so as to give the largest available cone. 

 A central stop to the condenser makes the markings 

 sharper. Its size can be calculated, according to Metzner 

 (986) as twice the calculated full cone of aperture necessary 

 to resolve the object, minus the aperture of the objective 



