TESTING MICROSCOPE OBJECTIVES, 147 
With the lower powers the artificial star is about the best test for 
determining this condition. With the glass now used by opticians 
for the construction of objectives, perfect achromatism has never 
been attained ; there being at best a secondary spectrum caused by 
the unequal dispersion of the various parts of the primary spectra 
of the flint and crown glass lenses. ‘These colors are a pale green 
and pale violet. Ifa well-corrected objective be focussed upon the 
mercury globule, and then brought without and again within the 
focal point, it will be seen that the circle of light has a colored 
border, and that when the globule is without the focus this border 
is of a pale green color, when within it a pale violet. ‘These colors 
indicate the best chromatic correction, and any variation will show 
itself by more or less change in the color. If undercorrection be 
present it is indicated by the green taking on a bluish cast, or, if 
very much undercorrected, a distinct violet is to be seen. In the 
place of the proper violet—that is when within the focus—a reddish 
violet, or even a brilliant red, is seen. Overcorrection is indicated 
when in the place of the green, a yellowish-green, brilliant yellow, 
or orange is seen; and in the place of the proper violet, a blue 
border is seen. This applies, not only to the mercury globule when 
used as stated, but to almost any opaque border of an object. With 
the higher powers, a very convenient way is to use a well-marked 
diatom, or, what in most cases is still better, a strongly marked 
podura scale. In any such objects as these, the dots or spines 
appear violet and the interspacing green when the chromatic cor- 
rection is right ; and in the case of under or over correction these 
colors give way to the colors already mentioned as occurring with 
the artificial star. 
The correction of spherical aberration is more important than 
that of color. By spherical aberration is meant that the rays of 
light which pass through different zones of the objective do not 
unite in one point in the eye-piece. If the peripheral rays meet 
before the central ones, the objective is said to be spherically un- 
dercorrected, while if the reverse be the case, overcorrection is 
present. With the artificial star the presence of spherical aberration 
is shown by the coma, or circle of light, expanding unequally when 
the globule is placed within the focus, or the same distance beyond 
the focus. Should the greater expansion of the coma take place 
when the distance between the objective and globule is increased, 
the objective will be undercorrected, but when the greater expansion 
is within the focus, overcorrected. 
I have here this evening a one-inch objective with which, by the 
aid of the artificial star, I shall practically illustrate the appearances 
seen with a well-corrected glass. The workmanship of this lens is 
superb, and, although of moderate angular aperture, it is much 
better in definition and resolution than many lenses of much wider 
