OPTICS. 
613 
the plates with the .frftoke .of a candle, he places a globule 
between the two holes, and tacks the plates together with 
two or three rivets. 
• Dr. Hooke ufed to take a very clear piece of glafs, and 
to draw it out into long threads in a lamp; then he held 
tliefe threads in the flame, till they ran into round glo¬ 
bules hanging to the end of the threads ; laftly, hav¬ 
ing fixed the globules with fealing-wax to the end of 
a.Hick, fo that the threads Hood upwards, he ground off 
the'ends of the threads upon a whetftone, and polilhed 
them upon a frnooth metal plate with a little putty. Mr. 
Stephen Gray tells us, (Phil. Tranf. N° 2.2,1., 223.) that, 
for want of a fpirit-lamp, he laid a fmall particle of glafs, 
about the fize of the intended globule, upon the end of a 
piece of charcoal; and, by means of a blaft-pipe, with 
the flame of a candle, he loon melted it into a globule. 
He thus made them indifferently clear, and the fmalleft 
very round ; but the larger, by rafting upon the coal, 
were a little flattened, and became rough on that fide. 
He therefore ground and polilhed ^ them upon a brafs 
.plate, till he reduced them to hemifpheres. But he 
found that the fmall round globules not only magnified 
more, but lhowed objects more diftindf, than the hemi¬ 
fpheres. 
By thefe methods may fpheres be made much fmaller 
than any lens; fo that the belt Angle microfcopes, or 
thofe which magnify the mod, are made of them. For, 
fuppofe the diameter of a fpherule to be of a digit, the 
diftance of its focus will be ; and therefore, its real 
diameter to its apparent one,, as -5*2 *l~itV i that is, as^ to 
8, oras 3 to 512 ; or, laftly, as 1 to 170. The lurface of 
an objeff, therefore, will be increafed by it in the pro¬ 
portion of 1 to 28,900, and its bulk in a ratio of 1 to 
4.,913,000. 
The fmalleft globules, and confequentl.y the greateft 
magnifiers, for microfcopes, that have yet been executed, 
\vere made by F. Di Torre of Naples, who, in 1765, fent 
four of them to the Royal Society. The largeft of them 
was only two Paris points in diameter, and was faid to 
magnify the diameter of an objeft 640 times; the fecond 
was the fize of one Paris point, magnifying the diameter 
1280 times; and the third, no more than one-half of a 
Paris point, or the 144th part of an inch in diameter, and 
was faid to magnify the diameter of an objedt 2560 times, 
and, Confequently, thefquare of fuch a diameter^,553,600 
times. But, fince the focus of a glafs globule is at the 
diftance of one-fourth of its diameter, and therefore 
that of the third globule of Di Torre, above mentioned, 
only the 576th part of an inch diftant from theobjedf, it 
muff be with-the utmoft difficulty that globules fo minute 
as thefe can he employed to any purpofe ; and Mr. Baker, 
to whofe examination they were referred, confiders them 
as matters of curiofity rather than of real ufe ; for expe¬ 
rience, fays he, has taught us, that thofe globules, which 
were at firft highly extolled and much fought after, admit 
fo little light, can (how only fuch an exceeding-minute 
part of any objedl, are fo difficult to be ufed, and ftrain 
the eyes fo much, that their power of magnifying, for 
want of due diftindtnefs, is rather apt to produce error 
than to.difcover truth, and therefore now they are very 
rarely employed. 
Microfcopes are of various kinds ; but chiefly, 1. Angle ; 
2. double or compound. We have alfo, 3. the reflecting 
roicrofcope ; 4. the folar microfcope; and 5. the rnicrofcope 
for opaque objedls; which, of courfe, are only modiflca- 
tions of the two flrft kinds. 
I. Single Microfcopes .—The mod Ample inftrument of 
this kind is reprefented on Plate X. at Ag. 1. AB is a 
little tube, to one of whofe bafes, BC, is fitted a plain 
glafs, to which an objedi, viz. a gnat, wing of an infedl, 
down, or the like, is applied ; to the other bafe, AD, at 
a proper diftance from the objedt, is applied a lens con¬ 
vex on both Ades, whofe femi-diameter is about half an 
inch ; the plain glafs is turned to the fun, or the light of 
Vol. XVII. No.-i-aoz. 
a candle, and the objeft is feen magnified ; and, if the 
tube be made to draw out, lenfes oiM life rent fpheres may 
be ufed. 
Fig. 2. reprefents aconvenient and pjeaftng inftrument..' 
A lens, convex on Both Ades, is inclofed in a cel] AC, 
and by a fcrew H is there faftened . through the pedeftul 
CD pafles a long fcrew, by means of which, and the 
female fcrew I, a ftyleor needle, fixed perpendicular to its 
extremity, is kept Ann at any diftance from the lens: in F, 
is a little tube, on which, and on the point G, the various 
objedis are tobedifpofcd : tints there may be lenfes of va¬ 
rious fpheres applied. 
But the microfcope which is found to anfwer the end 
beft, is Mr. Wilfon’s pocket-micVofeope, which has nine 
different magnifying glaffes. The body of this inftrument 
is reprefented at AA, BB, Ag. 3. and is made of ivory, 
brafs, or filver; it has three thin plates at EE, and a fpiral 
fpring of fteel wire H within it; to one of the thin plates 
of brafs is fixed apiece of leather, wood, or ivory, F, with 
a fmall furrow G, both in the leather and brafs to which it 
is fixed : in one ..end of this inftrument there is a long 
fcrew D, with a convex glafs placed in the end of it at C : 
in the other end, AA, of the inftrument there is a hollow 
fcrew, in which any of the magnifying glail’es are fcrewed 
when they are to be made'ufe of. The nine different 
magnifying glaffes are-all fet in ivory, eight of which are 
in the manner exprefied at fig. 4. The greateft magnifier 
is marked upon the ivory, in which it is let, with N° 1, 
the next N° 2, and fo on to N° 8 ; the ninth glafs is not 
marked, but is fet in the manner of a little barrel-box of 
ivory, to be held in the hand for the viewing of any 
larger objedi. Fig. 5. is a flat piece of ivory, commonly 
called a Aider, of which there are eight belonging to this 
fet of microfcopes, (though any’ohe who has a mind to 
keep a regifter of objedls may have as many of them as 
he pleafes) ; in each of them there are four holes, in 
which four or more objedls are placed between two thin 
glaftes, or talcs; when they are to be ufed with the greater 
magnifiers. ’ 
The mode of tiling the inftrument is this: A handle, 
fimilar to that at W in fig. 6. is made to fcrew on and off 
at pleafure : we mall fuppofe it now to be fcrewed upon 
the button S. You then take one of your Aiders, and 
Aide it betwixt the two thin plates of brafs at E, through 
the body of the microfcope, io that the objedl you intend 
to look upon be juft in the middle. Then you are to 
fcrew into the hollow fcrew in the end AA of the body 
of your microfcope the magnifying glafs- you intend to 
ufe ; which being done, put the end AA clofe to your 
eye, and, while you are looking through your magnify^ 
ing-glafs upon the objedt, you are to fcrew in or out the 
long-fcrew D, which, moving round upon the leather F, 
held tight to it by the fpiral wire H, will bring youf ob¬ 
ject to the true diftance ; which you will know by feeing 
it clearly and diftindtly : but, fince in the greater magni¬ 
fiers-you can fee but a fmall part of the .objedl, viz. the 
legs or claws of a flea; while you are looking upon any 
part of the objedl, if you take hold of the end of the 
plate or Aider, on which the objedl lies, and move it gent¬ 
ly, you may fee the whole objedl iuccefiiyely, or any 
part of it you pieafb ; and, if that part of the objedt 
you defign to look upon be out of the true diftance, re¬ 
member your endTcrew, D, can always bring k in, by 
fcrewing it one way or tiie other. A Ampler and more 
convenient method of mounting Angle microfcopes is,.to 
fix the feveral magnifying lenfes in a flat circular piece of 
brafs, which can be moved round a point as a centre, by 
the adtion of an endlefs fcrew upon the toothed circum¬ 
ference of the circular plate. 
_ After this manner may be feen ail tranfparent objects, 
dufts, liquids, cryftals of falts, fmall infedhs, fuch as fleas, 
mites, &c. Tf they be infedls that will creep away, or 
fuch objedls as one intends to keep, they may be placed 
between two of the regifter-glaffes; for, by taking out 
7 (with- 
