1898.] MICROSCOPICAL JOURNAL. 37 



t 



diatom slide may be placed on the plate carrier, and the 

 stage axis tilted 45 degrees if the stand is not rigid. 

 Then rest the glass plate on the metal stage. The slide 

 carrier may be slid up and down fifty times in ten sec- 

 onds by the right hand, while the left hand pushes the 

 glass stage plate across the field in an unvariable hori- 

 zontal course by impulses of one thousandth of an inch 

 or less. By this means not a speck can escape scrutiny. 

 Another valuable use of the glass stage plate is that 

 by placing a slide on it, on whose surface is a liquid con- 

 taining diatoms, a moving diatom once in a field of a 1-6 

 objective may be kept there and its movements studied 

 for hours at a time. The movable stage being of suita- 

 ble weight may be constantly shifted for long intervals 

 without causing a jar or tremor of the slide. The slide 

 itself is not touched after being put in position on the 

 plate. A small hemispherical condensing lens can be 

 easily attached to the under side of the plate by a liquid 

 contact, and a strongly lighted field can always be had 

 by turning the light from the concave mirror onto the 

 lens. 



On Double Color Illumination. 



It is possible with substage condenser and iris dia- 

 phragm to so light a diatom as 'to reveal the primary 

 structure in one color and the secondary in another. 

 Heretofore workers have used cones of light greatly ex- 

 ceeding the aperture of the objective or else cones very 

 much smaller than the aperture of the objective. The 

 former was on the dark ground principle — the latter in- 

 volving diffraction. But Mr. Rheinberg has found a plan 

 for getting rid largely of diffraction color effects and for 

 using any cone of illumination desired. Just as in low- 

 power color illumination on the dark ground principle, 

 he places in the substage condenser one of the ordinary 



