13 
is placed ten inches from the stage, and forms an image of 
itself in the focus of the instrument by transmitting a pencil 
of rays through a fine ;+,th objective secured to the stage. Itis 
viewed with 14th and C eye-piece of one inch focal length 
(power of 800 diameters), both objectives having been pre- 
viously adjusted for viewing an uncovered object. 
Result.—Nebulous field. Graduation (minutes) invisible. 
’ No index corrections of objectives removed the nebulosity. 
The ith observing glass was of average quality. Residuary 
aberration most decided. 
Experiment 12.—The positions of the two objectives are 
reversed. ‘The image formed by the 1th is viewed by the ~!;th 
with equal power. 
Result.—Nebulosity is diminished greatly. 
Deduction.—Aberration of the -!,th better corrected than 
that of the 1th. 
Experiment 13.—Mercurial globules of various sizes placed 
on black cloth now replace the watch-dial at ten inches 
from stage. 
Result.—Beautiful sharp images of the window visible in 
the particles of mercury miniatured by the -;th stage objective 
arranged as in Experiment 11 like a condenser. 
Deduction—The difficulty of observing minute globules 
placed upon the stage entirely avoided. A new order of tests 
is secured of any degree of minuteness and brilliance. This 
method is recommended as avoiding the inconvenience and 
practical difficulty of viewing the image of a brilliant disc 
upon a globule of mercury almost touching the objective, 
and impossible with the immersion lens. 
For ordinary purposes of testing, nothing can be more 
simple than employing artificial spectra formed by either 
objectives or lenses. If the former are selected, then the 
observing lenses must be of known character and quality. 
If the latter, then their especial qualities must also be known. 
Achromatism, so-called, with spherical error or planatism, 
gives a pale grey-yellowish hue and a perspective resembling 
a view through horn or any slightly opaline medium. The 
details of the dial or other object cannot be distinguished. 
The stage image of a watch resembles fig. 10. 
Chromatism with fine aplanatism is in every case better 
seen by using a bright flame or solar rays. If a small, bril- 
lant camphine lamp be placed a few feet from the stage and 
in the instrumental axis, when the apertures are very large, a 
gorgeous halo of rich colouring in the order of dispersion 
irradiates the outline of the flame imaged by the objective 
used as a condenser. 
