t be the common Focus of
the Speculum ABDC, and of a Plano-convex Eye-glass H, through which
those Rays must pass to the Eye. And let the Rays at their coming out of
the Glass pass through a small round hole, or aperture made in a little
plate of Lead, Brass, or Silver, wherewith the Glass is to be covered,
which hole must be no bigger than is necessary for Light enough to pass
through. For so it will render the Object distinct, the Plate in which
'tis made intercepting all the erroneous part of the Light which comes
from the verges of the Speculum AB. Such an Instrument well made, if it
be six Foot long, (reckoning the length from the Speculum to the Prism,
and thence to the Focus T) will bear an aperture of six Inches at the
Speculum, and magnify between two and three hundred times. But the hole
H here limits the aperture with more advantage, than if the aperture was
placed at the Speculum. If the Instrument be made longer or shorter, the
aperture must be in proportion as the Cube of the square-square Root of
the length, and the magnifying as the aperture. But it's convenient that
the Speculum be an Inch or two broader than the aperture at the least,
and that the Glass of the Speculum be thick, that it bend not in the
working. The Prism EFG must be no bigger than is necessary, and its back
side FG must not be quick-silver'd over. For without quicksilver it will
reflect all the Light incident on it from the Speculum.

[Illustration: FIG. 29.]

In this Instrument the Object will be inverted, but may be erected by
making the square sides FF and EG of the Prism EFG not plane but
spherically convex, that the Rays may cross as well before they come at
it as afterwards between it and the Eye-glass. If it be desired that the
Instrument bear a larger aperture, that may be also done by composing
the Speculum of two Glasses with Water between them.

If the Theory of making Telescopes could at length be fully brought into
Practice, yet there would be certain Bounds beyond which Telescopes
could not perform. For the Air through which we look upon the Stars, is
in a perpetual Tremor; as may be seen by the tremulous Motion of Shadows
cast from high Towers, and by the twinkling of the fix'd Stars. But
these Stars do not twinkle when viewed through Telescopes which have
large apertures. For the Rays of Light which pass through divers parts
of the aperture, tremble each of them apart, and by means of their
various and sometimes contr