NOTES AND MEMORANDA. 
89 
adjusting the screws to the thickness of the pane in snch a manner 
that the pane will be held with sufficient security, and, at the same 
time, permit it to be instantly removed when required. This simple 
arrangement it will be seen admits of the glass being placed in posi- 
tion or removed therefrom at a moment's warning. 
The table being now properly placed before an open window in 
such a manner that the pane of glass shall intercept the solar beam, 
and the microscope stand being placed near to and behind the same, 
the microscopist is ready to use monochromatic sunlight with quite the 
same facility as diffused daylight ; while by the arrangement described 
the entire stand is enveloped in an atmosphere of blue light. After, how- 
ever, observing for a few moments, owing to the movement of the sun, 
the entire table will require to be moved. This is scarcely the work 
of a moment, and need not in any way disturb the " resolution " of the 
most difficult and severe tests. 
A little practice will be found of value in determining the amount 
of solar beam to be used ; and this amount will vary with the strength 
of the sunlight, as also with the objective and eye-piece used. As a 
general rule the amount; of light should be no greater than what the 
eye can comfortably bear, anything like a resplendent field should be 
avoided. I find it preferable to use the mirror ; a slight touch of this 
will quickly produce the effects desired. 
The obliquity of the illumination will of course depend (when 
using the low-angled objectives) on the objective used, but with modern 
wide-angled objectives excessive obliquity is to be avoided, and in no 
case should the illumination be so oblique as to badly distort the 
image. With Mr. Tolles' new 4 system ^th and y^^^th objectives a 
beam proceeding from the mirror to the object, making an angle of 
40° or 50° with under surface of slide, is quite sufficient to show 
the most difficult tests known ; although with the objectives named a 
much more oblique angle can be used without sensibly distorting the 
image. 
With the arrangement above described any ordinarily good dry 
■|th ought to show A. pellucida's transverse strisa strongly and with 
little trouble. A very commonplace yV^h of my own and which is 
greatly inferior to my new Tolles' will give me the transverse striae on 
A, pellucida splendidly and without trouble. 
All observations in the line of advanced research should be made 
with monochromatic light. In comparison, diffused daylight or the 
best artificial light sinks into insignificance. 
Do not let the beauty of the "resolution" with the blue light 
result in attaching undue importance to an inferior objective ; for a 
very ordinary glass under these favourable conditions will seem to 
perform finely. For the test of objectives use always artificial light 
and halsam mounts, without condenser. A modern wet -y-^th. should 
thus go readily and easily through the " Moller's Probbe Platte," 
showing strongly the striss on Nos. 18, 19, and 20, either with "D" 
or ^-inch solid eye-pieces. In short, a good modern immersion yVth 
should show with lamplight any known natural test, even when halsam- 
mounted. 
