148 THE MICROSCOPICAL NEWS. 
angle. By the aid of the back system of a cheap English one- 
fourth-inch objective, I shall illustrate the irregularity of outline of 
the coma, which indicates a want of true form or curves, of the 
lenses. 
Angle of aperature is of great importance. By this is meant the 
angular breadth of the cone of light received from the object by the 
objective. This is to be determined by direct measurement. As 
convenient a method as any, is by using a stand which has a grad- 
uated rotating base, or graduated swinging substage bar. The 
objective should be put upon the stand and accurately focussed, the 
stand being in a horizontal position. A lamp is then placed several 
feet distant, and, by turning the stand upon the base, the aperture 
is ascertained, the light being allowed to divide the field equally at 
each extreme of the angle. In case the measurement is to be made 
by the aid of the substage bar, this may be made to carry a small 
candle or lamp, which should be diaphragmed to a narrow slit. ‘The 
measurement, as before stated, should be made from the point at 
which the light bisects the field. When it is desired to measure 
balsam aperature, or the angle of the rays in Canada balsam, it is 
necessary to use the proper immersion fluid, and to have a hemis- 
pherical lens placed under the object (the object, of course being 
balsam mounted) whose centre of curvature is in the plane of the 
object. The readings upon the base or substage bar then indicate 
the angle within the hemispherical lens ; and if this be of the same 
glass as the front lens of the objective, the true glass angle, or, as it 
is more commonly called, the balsam angle, becomes known. 
I think it may not be unjust to say that many of the objectives of 
our best makers will fall considerably short of the aperture claimed 
for them. A recent + for which an angle of 130° in balsam, was 
claimed, only measured 125°; a ie fine ;'5 for which a 140 was 
claimed, only gave 116°; and a 75 claimed ‘to have 105,, only gave 
An objective may have a very wide angle of aperture and still 
be inferior to that of another objective of less angle. In such cases 
the trouble is generally due to a want of proper correction for chro- 
matic and spherical aberration. In measuring aperture it is always 
well to select some test-object which shall be more or less difficult 
to resolve with the objective to be measured. It should then be 
focussed and carefully watched as the stand or mirror bar is turned, 
until the very best resolution is attained. By increasing the obliquity 
of the light it may then be seen whether the extreme rays which 
traverse the periphery of the objective are really of any use. For 
example, in the + already mentioned the light will enter the objective 
at 125°, but the light which forms the image only enters at T20°% 
hence the objective is no better than one properly corrected having 
120. 
