254 On the Advaneing Aplanatic Power [Mirus Nov 1 toro. 
III.—On the Advancing Aplanatic Power of the Microscope, and 
New Double-Star and Image Tests. By G. W. Royston-Picort, 
M.A., M.D. Cantab., M.R.C.P., F.C.P.8., F.R.A.S., &., &e. 
Part J. 
Tue continually expanding range of the application of this charm- 
ing instrument appears to justify the incessant labours now under- 
taken by optical artists and enthusiastic amateurs for its continual 
refinement and improvement. Less than half a century ago, a mere 
toy, it now ranks with the most delicate instruments of physical re- 
search ; and none appears to have such an array of inventions for 
heightening even its finest effects. 
Yet the whole excellence of the instrument depends upon the 
veracity of its readings, the truthfulness of its interpretation ; that is, 
the power of the observer to distinguish true from false phenomena, 
and appreciate, and if possible eliminate, actual sources of error. 
At present the lovers of this instrument are somewhat divided 
in their opinions as to its present merits in the higher departments 
of microscopical research. 
A large majority of observers adhere to the belief in the absolute 
correction of any appreciable errors; in the perfection, in fact, of 
our modern instruments. 
A writer of acknowledged amateur skill in constructing objec- 
tives (the first to form a ;'5th of an inch focal length) thus expresses 
his opinions, no doubt pleasantly reflected by numerous vendors of 
objectives of the first class :—“ Objectives from the hands of careful 
and experienced makers have all been constructed on the globule 
test, and are not sent forth till every error of workmanship, centering, 
state of oblique pencils, achromatism, and spherical aberration, are 
all absolutely corrected ; for this discovers the least fault of either, 
when all others fail.” * 
And speaking of the immersion system, the same writer tells 
us— 
“Not either the water or glass-cover has introduced a single 
new element of correction, and will not, therefore, bear out the 
following assertion in the paper referred to :—‘ The extraordinary 
difference between the performance of the hydro-objective and of the 
pneumo-objective (the plate of air making enormous differences in 
the aberration of the glasses)} must make it apparent to common 
sense that our old-fashioned glasses must be wrong somewhere’ 
(the sentence may be completed properly here), ‘and if not in fail- 
ing to converge the image of a point to another point, I know not 
where to find it, ¢.e. in aberration.’ ” 
* M. M. Journal, p. 302. 1870. 
+ See Plate LX. on the aberration of the two systems. 
