THE YOUNG SCIENTIST. 
9 
feet correetion to be obtained. The light 
requires to be set as close as it can be, 
and, of course, in the highest powers, 
where there is little distance in front, it 
must be very oblique ; but this is of no 
consequence, as it is not the globule itself, 
but the spot of light reflected from it, 
that is required to be seen. 
The lens to be tested is adapted to the 
microscope, having the ordinary Huy- 
ghenian eye-piece. On placing the globule 
either in or out of focus, the luminous 
point expands into a ring. If the object- 
glass is under-corrected for color, as in 
a single lens, the bright ring appears 
within the focus, the outer margin is red, 
and the inner circle green. If the lens is 
over-corrected, the bright ring appears 
ivithout the focus, with the colors in the 
same order as before. A practical know- 
ledge only, derived from these appear- 
ances, can determine the amount of con- 
cavity to be given to the flint, or differ- 
ence of convexity in the crown, for ob- 
taining the desired correction ; but even 
In the most experienced hands it generally 
involves several alterations to secure per- 
fect achromatism. When this is cor- 
rected as far as practicable, a pale-green 
color only is perceptible beyond the focus. 
This arises from the secondary spectrum, 
or relative difference in the width of the 
prismatic color spaces of the crown and 
flint, and seems to be a variable condition, 
according to the composition of the glass 
employed. 
Though correction for spherical aberra- 
tion is intimately related to that of color — 
a single lens when Anally achromatised, 
being also nearly free from spherical 
error ; yet, in a combination of three pair, 
when matched so as to be achromatic, 
this may be so -considerable as to render 
the object-glass useless, and is oftentimes 
exceedingly troublesome to remedy. The 
error may arise from an improper pro- 
portion between the relative loci of the 
lenses— as the back being too long. I 
have before stated that, in the form that 
I have advocated, the spherical aberration 
is mainly corrected by giving thickness 
to the front lens, and by properly ad- 
justing the distance between them. In a 
glass spherically under-corrected the 
light from the globule is greatest within 
the focus, and when set out of focus 
speedly vanishes and becomes diffused; 
in the case of spherical over-correction 
the contrary appearances result. When 
the relative distance of the lens is rightly 
adjusted, the light spot expands equally, 
and is of the same intensity, for a short 
distance on either side of the focus, in 
which the globule should appear with a 
clear bright margin. The object-glass is 
now in a proper condition for testing errors 
of construction and workmanship. 
To examine the condition of the oblique 
pencils, and consequent flatness and dis- 
tinctness throughout the field, a small 
globule is selected, and brought to the 
edge, using the lowest eye-piece; if the 
bright point in the centre of the globule, 
when a little out of focus, approaches to 
the inner side of the concentric light-rings, 
as in Fig. 20, it is termed "outward coma," 
and indicates that the front incident sur- 
face of the back triple is too convex. If, 
on the other hand, the bright spot is on 
the outer side of the rings, or next the 
margin of the field of view, there is "in- 
ward coma," which shows that this same 
surface is too flat. I have previously re- 
marked that this curve has a powerful 
effect on the flatness of field and perfec- 
tion of oblique pencils, and for these no 
other correction is generally requisite 
than an alteration in this radius. 
Before the glasses are finally cemented 
in their cells, they should be carefully 
tested for centering; for this purpose 
a very minute globule is selected, and 
placed exactly in the centre of the field. 
If the bright spot appears eccentric, 
with the rings thus (Fig. 21), the pair of 
lenses which occasion the error should be 
shifted on each other while ww^m enough 
to cause the Canada balsam by which 
they are cemented together to yield, till 
on repeated trial the error is corrected. 
This is important, as the least fault of 
centering materially impairs the perform- 
ance of an object-glass. But with the 
precautions that I have adopted in the 
construction, already explained, errors of 
centering cannot occur. 
There is yet one other globule test for 
object-glasses, to indicate accuracy of 
