Thus, a telescope, having a very large aperture, though of
an erroneous figure, will shew nebulae, and clusters of stars,
totally invisible by a small telescope of the most perfect
kind ; and the latter again will, in its turn, shew double
stars and other minutiee, which the former would be utterly
incompetent to render manifest* In the same manner, an
engiscoi)e of a small angle of aperture will define, and
shew very exactly, an enamelled dial plate, the outline of
the brilliant scales of a variety of beetles, animalcules, and
an innumerable quantity of common objects, too tedious
to mention; but it will utterly fail upon the lined objects;
while another, having a large angle of aperture, though
replete with aberration, will nevertheless shew many of
these, (though not so well, of course, as if aplanatic and
achromatic ;) while, if tried, on a dial plate, or a piece of
diamond beetle, &c. it will exhibit its imperfections in a
most glaring manner, ahd shew a total deficiency of what
I call defining power. There are objects which at once
serve to exhibit the perfection of a microscope, or engiscope,
both as to defining and penetrating power, provided the
manner in which they are shewn is duly attended to.
The following aphorisms will serve to express my views
relative to the subjects of this memoir.
1. A test is an object which serves to render sensible
both the peifection and imperfection of an instrument, as
to defining and penetrating power.
