1 1 Mi c r o s c o p i c al E ssayas.
the refraflive power of water is lefs than that o f glafs, thefe
globules do not magnify fo much as thole o f the fame .fize,
which are made ,o f glafs : this was alfo contrived by IVfr, .Gray,
The fame ingenious author invented another water, microfcope,
confiliing o f two drops o f water, feparated in part by a thin brafs
plate, but touching near the center; which were thus rendered
equivalent to a double convex lens, o f unequal convexities.
Dr. Hooke defcribes a method o f ufing the Angle microfcope,
which feems to have a great analogy to the foregoing methods
o f Mr. Gray. I f you are defirous (he fays} o f obtaining a microfcope
with one Engle refraction, and confequently capable- of
procuring the greateft clearnefs and brightnefs any one kind
o f microfcope is fufceptible o f ; fpread a little o f the fluid you intend
to examine on a glafs plate, bring this under one - o f your
microfcopic globules, then move it gently upwards, till the fluid
touches the globule, to which it will foon adhere, and that fo
firmly, as to bear being moved a little backwards or forwards,
By looking through the globule, you will then have a perfect view
o f the animalculm in the drop. *
Having laid before the reader the principal improvements that
have been fuggdled or made in the Engle microfcope, it remains
only to point o u t thole inftruments o f this kind, which, from the
mode in which they are fitted up, feem belt adapted for general
ufe ; the peculiar advantages of which, as well as the manner of
ufing them, will be defcribed in the third chapter o f this work.
Fig.
Iluoke’s Lectures and Conjectures, p. 98,
pig. W Plate VI. reprefents that which was ufed by M. Lyon-
net for diflefling the cbflus.
Fig. i. Plate VII. B. Thé aquatic microfcope ufed by Mr.
Ellis M inveftigating the natüre of Coralline, and recommended
by Mr. Curtis to botanills, in his Flora Londinenfis.
Fig. 1. Plate VI. A botanical microfcope, contrived by Dr.
Withering.
Fig. 2. Plate V I. A botanical microfcope, by Mr. B. Martin.
Fig. 5. Plate VI. The tooth and pinion microfcope, which is
now- generally fubftituted in thé room of Wilfon s.
Fig. 8. Plate VIII. A botanical magnifier.
Fig. 7. Plate VIII. A different kind of botanical magnifier.
A compound microscope, as it cónfifls of two, three, o r
more glafles, is more eafily varied, and is fufceptible o f greater
changes in it’s confiruHion, than the fingle microfcope. Thé
number of the lenfes, of which it is formed, may be incréafed or
diminilhed, their rêfpëftive pofitions may be varied, and the
form in which they are mounted be altered almoft ad infinitum.
But among thefe varieties, fome will be found more dêferving o f
attention than others; it is of thefe only we Ihall fpeak in this,
placé. '
The