THE MONOCULAR MICROSCOl'E 59 



2. A diaphragm, about 14 millimeters across, in the drawtube, 

 about 60 millimeters below the top, may be necessary to cut off 

 reflections (Nelson). 



3. Rackwork on the drawtube was an advantage in using high 

 dry objectives without correction collars (Nelson, Coles). But 

 the use of a long drawtube can be avoided by choosing proper 

 cover-glass. 



4. With an incorrect cover-glass thickness the tube length 

 should be carefully adjusted; for, with certain oil-immersion 

 objectives, a millimeter in error may deteriorate the image, 

 sHghtly but perceptibly. With water-immersion objectives 

 especially, the crispest image is only obtained with correct 

 cover-glasses. 



5. Much (or any) changing of eyepieces may be avoided. It is 

 often better to change objectives. Too low eyepieces show 

 curvature of the field. 



6. The wide microscope tube seems only useful as allowing 

 larger eyepieces, and hence larger fields (if these are wanted). 



7. Both a diaphragm in the drawtube and a wide tube for 

 photography are rendered unnecessary by using a source of light 

 equal to the source-field. 



8. For routine work, it seems preferable to omit the drawtube, 

 as is now done by some optical firms, and thus avoid errors from 

 its neglect or misuse. In this case, we may change the cover- 

 glass instead of changing the tube length, when we find a cover- 

 glass too thick or too thin. 



9. The monocular with a drawtube is suited for the expert 

 microscopist to correct low objectives for uncovered objects, and 

 to correct other objectives (including oil-immersion objectives) 

 for different thicknesses of cover-glass, or for different optical 

 tube lengths. But, by such correction, images are not obtained 

 quite equal to the results with the correct cover-glass thickness of 

 0.17 millimeter. 



10. Distance spectacles, if any, should be used on the monoc- 

 ular microscope. For, with reading glasses, the eye would Ix; 

 looking through a part of the spectacle lens not the center. 



