Diatomaceous Tests. By Adolf Schulze. 
47 
minator so that its polished off face looks as it were to the right, 
the edge of the wick facing the mirror or rectangular prism at 
the tail - piece. After having racked up the reflex illuminator 
until its plane top is level with the stage, and after having 
centred the dot by means of a |-inch objective, I put a few 
drops of castor-oil, or better, of glycerine, which is easier cleaned 
off by water, on the top of the illuminator, taking care that 
no air-bells are formed, and that none of the fluid flows down 
the j)olished off face. I then illuminate as for dark ground, and 
focus for the object, generally first for one of the larger diatoms 
on the Platte, using of course one of those large-angled immersion 
lenses which I classed among No. 3. By lowering the reflex 
illuminator or the immersion paraboloid from the one-twentieth to 
the one-thirtieth of an inch, the ground becomes light, the red 
appearing to the right and the blue of the spectrum to the left of 
the field. By a little more focussing of the condenser and adjust- 
ing of the mirror or rectangular prism, or by altering the position 
of the flame slightly, the best result is soon obtained. I invari- 
ably use the edge of the flame for throwing light on the mirror 
or on the rectangular prism, and often I remove the latter alto- 
gether and place the flame in the axis of the field lens of the 
reflex illuminator. A white field is best obtained in this way. 
Sometimes I place the microscope horizontally on a pedestal from 
4 to 6 inches high above the level of the table, and I find this a 
very convenient position for observing. By interpolating a bull’s- 
eye condenser, convex side towards flame, and by using the broad 
side of the wick, the prismatic colours disappear and the field 
becomes white. By turning the microscope a little to the right or 
to the left on its vertical axis the best illumination is often very 
easily secured. The light from the reflex illuminator should fall 
under an angle of about 6° against the midrib of Am'phifleurob 
jpell'cida. 
The reflex illuminator may also be used for the resolution of 
lined tests by transmitted light, especially sunlight when the 
objects are mounted dry and on the cover. In this way the lines 
on Amjfhifleura pellucida can be shown beautifully. I have also 
found that by transmitted sunlight the reflex illuminator may be 
used for the resolution of either balsamed or dry lined tests 
mounted on cover, without the use of any liquid medium whatever 
between the top of said apparatus and the under side of the slide. 
The value of this method is not confined to lined diatomaceous 
tests, but may become of great service for biological and other 
researches. 
The immersion objectives, with which I have succeeded in 
resolving the finest specimen of Ani'phi'pleura ]yellucida in balsam 
were those of Powell and Lealand’s on the new formula, the one- 
