EXAMPLES OF PROCEDURE. 
95 
must also be focussed with even greater care than for 
the less difficult work. For the finest tests—with 
diatoms, for example—an immersion condenser should 
be used, i.e., if the objects are mounted in media of 
high refraction index. Nothing is gained by the use of 
an immersion condenser on dry-mounted objects. And 
for dry objects it is necessary to make sure that the 
object treated is in contact with the cover-glass, which 
can be ascertained by the use of a “ vertical illumi¬ 
nator,” such as is, or used to be, made by Messrs. Beck. 
A very great assistance in the most critical work will 
be found in the use of a “ monochromatic ” light-filter. 
A “ blue-green ” filter has been made by Mr. Gifford, 
an amateur of great ability, and Air. Sanger Shepherd 
has made a very excellent one for the writer. 
In focussing with the high powers on the plain glass 
of the camera, the “spectacle-lens” focussing eye-piece 
of Dr. Bousfield (see page 41), will be found superior to 
the Ramsden or Aplanat. 
As the exposure increases with the magnification, the 
greatest care must be exercised to avoid tremor in high 
power work ; and the heat given off by the radiant, be 
it oil or lime-light, often causes displacement of the 
parts of the microscope, and consequent blur of the 
image ; it is, therefore, well to let the light burn in its 
place for some time previous to making an exposure, 
examining the image just before putting in the plate. 
In cold weather, this is specially to be observed. 
Unless the room is specially steady, no walking about 
in it should be allowed during exposure. 
