614 
OPTICS. 
(with the point of a pen-knife or fmall plyers) the ring 
that keeps-in the glafles where the objedt lies, they will 
fall out of themfelves ; fo that you may lay the object 
between the two hollow fides of them, and put the ring 
in as it was before : but, if the objedts be duffs or liquids, 
a fmall drop of the liquid, or a little of the dull, laid on the 
outlide of the glafs, and applied as before, will be feen 
very eafily. 
As to the firft, fecond, and third, magnifying glafles, 
being marked with a -f- upon the ivory in which they are 
fet, they are only to be ufed with thofe plates or Aiders 
that are alfo marked with a -j-, in which the objedls are 
placed between two thin talcs ; becaufe the thicknefs of 
the glafles in the other plates or Aiders hinders the ob¬ 
ject from approaching to the true diftance from thefe 
greater magnifiers. But the manner of ufing them is the 
fame with the former : only remember to be careful, when 
you put in or pull out the plate or Aider on which the 
object lies, or move it from one objedt to another, not 
to let it rub your magnifying glafs ; which is done by 
unfcrewing a little the end-fcrew D, when you put in 
or pull out your plate, or move it from one object to 
another. 
For feeing the circulation of the blood at the extremi¬ 
ties of the arteries and veins in the tranfparent parts of 
fifties’ tails, &c. there are two fliort glafs tubes, the one 
bigger and the other lefs, in which the fffh is to be put; 
when thefe tubes are to be ufed, you are to unfcrew the 
eaul-fcrew D in the body of the microfcope, until the 
glafs tube can be received eafily into that little cavity G 
of the brafs plate fattened to the leather F, under the 
other two thin plates of brafs at E. When the tail of 
your fifli lies flat to the glafs tube, fet it oppofite to your 
magnifying-glafs ; and, by fcrewing in or out your end- 
fcrew D, as is faid before, you may eafily bring it to the 
true diftance, and fee the blood circulate with great 
pleafure. 
If you would fee the blood circulate in a frog’s foot, 
choofe fuch a frog as will juft go into your tube ; then 
with a little ftick expand the hinder foot of the frog, and 
apply it clofe to the fide of the tube, obferving that no 
part of the frog hinders the light from coming on its foot; 
and when you have it at thejuft diftance, by means of the 
fcrew D, as above faid, you will fee the rapid motion of 
the blood in its veflels, which are very numerous, in the 
tranfparent thin membrane that is between the frog's 
toes. For this objedt the fourth and fifth magnifiers will 
do very well ; but you may fee the circulation in the tails 
of water-newts with the fixth and feventh glafles, becaufe 
the globules of the blood of thofe newts are as big again 
as the globules of the blood of frogs or fmall fifti, as has 
been taken notice of in M° 280 of the Philofophical 
Tranfadtions. The circulation cannot fo well be feen 
by the firft, fecond, and third, magnifiers, becaufe the 
thicknefs of the glafs tube in which the fifli lies hinders 
the approach of the objedt to the focus of the magnify¬ 
ing glafs. 
The Jingle opaque micrrfcope, fig. 6, remedies the in¬ 
convenience of having the dark fide of an objedt next the 
eye, which formerly was an infurmountable objection to 
the making obfervatiqns on opaque objects with any con- 
fiderable degree of exadtnefs or fatisfadtion ; for, in all 
other contrivances commonly known, the nearnefs of the 
inftrument to the object (when glafles that magnify much 
are ufed) unavoidably overlhadows it fo much, that its 
appearance is rendered obfcure and indiftindt. And, not- 
withftanaing ways have been tried to point light upon an 
objedt, from the fun or a candle, by a convex glafs placed 
on the fide thereof, the rays from either can be thrown 
upon it in fuch an acute angle only, that they ferve to 
give a confufed glare, but are infufficient to afford a clear 
and perfect view of the objedt. But in this microfcope, by 
means of a concave fpeculum of (liver highly poliflied, in 
whofe centre a magnifying lens js placed, fuch a ftrong 
and-direct light is reflected upon the objedt, that it may 
be examined with all imaginable eafe and pleafure. The 
feveral parts of this inftrument, made either of brafs or 
filver, are as follow : 
Through the firft fide A, paffes a fine fcrew C, the other 
end of which is fattened to the movable fide B. D is a nut 
applied to this fcrew, by the turning of which the twx> 
fides A and C ate gradually brought together. N is a 
fpring of fteel that feparates the two fides when the nut 
is unfcrewed. P, P, P, are joints which turn eafily any 
way. F is a piece of brafs, turning round in a focket, 
whence proceeds a fmall fpring-tube moving upon a rivet; 
through which tube there runs a fteel wire, one end 
whereof terminates in a (harp point G, and the other with 
a pair of pliers H fattened to it. The point and pliers are 
to thruft into, or take up and hold, any infedt or obje< 5 t; 
and either of them may be turned upwards, as belt fuits 
the purpofe. Upon the end L there is a fcrew, upon 
which the glafs fet in the barrel-box may be lerewed. 
When the other glafles are to be ufed, there is a ring R 
of brafs, to be fcrewed on the end L, into which ring all 
the other glafles may be fcrewed. A concave fpeculum 
of filver, poliflied as bright as poffible, in the centre of 
which is placed a double convex lens, with a proper aper¬ 
ture to look through it, is made to fit the fame brafs ring, 
and to fcrew into it at pleafure. There are four of thefe 
concave fpecula of different depths, adapted to fourglafl.es 
of different magnifying powers, to be ufed as the objedts 
to be examined may require. M is a round objedt-plate, 
one fide of which is white and the other black. The in¬ 
tention of this is to render objedts the more vifible, by 
placing them, if black, on the white fide, or, if white, on 
the black fide. A fteel fpring N turns down on each fide, 
to make any objedt faft ; and ifluing from the objedt-plate 
is a hollow pipe, to fcrew it on the needle’s point G. A 
fmall box of brafs, with a glafs on each fide, contrived to 
confine any living objedt in order to examine it, is fome- 
tiifies added : this alfo has a pipe to fcrew upon the end 
of the needle G. W is a turned handle of wood, to fcrew 
ifito the inftrument when it is made ufe of. 
When you would view any objedt with this microfcope, 
fcrew the fpeculum, with the magnifier you think proper 
to ufe, into the brafs ring R. Place your objedt, either 
on the needle G in the pliers H, on the objedt-plate M, 
or in the hollow brafs-box O, as may be molt defirable ; 
then, holding up your inftrument by the handle W, look 
againlt the light through the magnifying lens; and by 
means of the nut D, together with the motion of the 
needle, by managing its lower end, the objedt may be 
turned about, raifed, or depreffed, brought nearer the 
glafs, or removed farther from it, till you find the true 
focal diftance, and the light be feen ftrongly refledled 
from the fpeculum upon the objedt, by which means it 
will be Ihown in a manner furprifingly diftindt and clear; 
and for this purpofe the light of the ficy or of a candle will 
anfwer very well. Tranfparent objedts may alfo be viewed 
by this microfcope ; only obferving, that, when fuch come 
under examination, it will not always be proper to throw 
on them the light refledled from the fpeculum j for the 
light tranfmitted through them, meeting the refledled 
light, may together produce too great a glare. -A little 
pradtice, how’ever, will fliow how to regulate both lights in 
a proper manner. 
For the convenience of thofe who would draw, or make 
any {ketches ordefigns of, microfcopical objedls, they may 
alfo have a pedeftal, to fix the two inftruments above de- 
fcribed, and make them ftationary to any convenient light. 
This pedeftal may be placed on a table ; and, after the 
objedt and fight are fixed, as many perfons as pleafe may 
view the objedt, without any trouble or difficulty in find¬ 
ing the light. (Phil. Tranf. abr. vol. iv. p. 199.) 
Single Microfcope bp Reflection .— Mr. Baker in his trea- 
tife of Microfcopes, vol. i. has defcribed an invention for 
fixing the pocket-microfcope of Wilfon, and giving light 
to it by a fpeculum. For this purpofe, A, fig. 7, is a fcroll 
of brafs fixed upon a round wooden bafe B, or mahogany 
drawer 
