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SCIOPTICON MANUAL.
whose flatness of field in the table microscope leaves noth¬
ing to be desired in that direction, are entirely unsatisfac¬
tory when used in the gas microscope.
One of the most influential causes of this we shall notice
presently, but we will here only remark that, as the result
of a large experience, we have become convinced that one
must be contented with a moderate amount of success in
this direction, and not expect what is, at present, at all
events, impossible.
The second great defect that we encounter in the use of
the microscopic lens for projection, is the irregularity of
distribution of light upon the screen.
By reason of this we may have a field of light with a
small bright area at the centre, rapidly fading off into dark¬
ness, with no well-defined margin.
The causes of this are, among others, the confusion or
want of accurate concentration of the cone of rays from the
condensers, and the smallness of the objective, causing it to
cut off oblique or marginal rays more or less, according to
their obliquity. To remedy this difficulty we can work in
two directions.
In the first place, we may improve the spherical correc¬
tion of the condensers, or the concentrated character of the
source of light. The first of these improvements has
already been carried to its practical limit in the best sort of
condensers, and the second involves the use of the electric
light or of sunlight.
In the second place, any increase in the diameter of the
microscopic lenses, without a corresponding increase in their
actual length, insures a great gain as regards the equal
illumination of the field.
With this view alone, therefore, a simple uncorrected or
single corrected microscopic lens, such as accompanies the
regular gas or solar microscopic attachment made for the