3 
surement. I propose also to give some striking examples of 
true, false, and natural definition. 
In the second part of this paper I hope to have the honour 
of laying before the readers of this Journal an account of new 
modes of correcting the objective by additional lenses, which, 
by traversing the axis of the instrument, operates as a searcher 
for aplanatism to detect the most perfect aristokratic locus for 
producing the finest final focal image, a contrivance which 
changes the state of the aberrations, both spherical and chro- 
matic, and introduces the new colour-test for high-power 
definition, at the same time that it very considerably deepens 
focal perspective and enhances magnifying power and in- 
creases the intensity of illumination. 
The same instrument also furnishes a means, in many 
cases, of applying a correction to objectives unfurnished with 
a screw-collar adjustment. 
Lastly, some researches in the effect of the immersion 
system, and its further improvement. 
The superiority of the immersion system, so much used on 
the Continent—it being far more easy to construct a good im- 
mersion objective than our English first-class glasses—should 
stimulate the inquiry into the causes of defect in aéro-objec- 
tives; indeed, notwithstanding the greater brilliance of the 
hydro-lens, it is still highly desirable to adhere to the dry 
form. It appears to the writer that the latter is far prefer- 
able as regards general utility and adaptability, as the im- 
mersion lens is necessarily inconvenient, in some cases im- 
practicable, and in many destructive. 
I may be permitted here to remark that the most important 
part of the instrumental adjustments is the minute interval 
by which the front sets of glasses are separated. Commonly 
employed to compensate for varying thickness of the usual 
“‘ glass cover” applied to protect and secure minute objects 
upon the “ slide,’ a screw collar, divided into fifty parts, 
regulates to the 10,000th of an inch this important interval, 
upon a noyel co-operation, of which these researches are 
intended principally to bear. 
The prospect of further advancement in the power and 
precision of the microscope is now bright and full of en- 
couragement. It is, however, almost necessary to start de 
novo. Many accredited facts must be relinquished. Several 
of the most trustworthy tests have now become antiquated 
and delusive. But considering the relations of this instru- 
ment to the arts, science, and jurisprudence, too much im- 
portance can hardly be attached to the uniformity, constancy, 
and fidelity of its readings. 
