OPT 
3. To adjuft the lenfes to their focal diftance : This is 
effected by turning the pinion a, the eye being at the ftme 
time'at the eye-piece L. The grey glafs is often placed 
before the large lenfes, while regulating the guide for the 
eye, and adjlifting for the focal diftance. 
If the obferver, in the procefs of his examination of an 
object, advance rapidly from a fliallovv to a deep magnifier, 
he will fave himfelffome labour, by pulling out the inter¬ 
nal tube at O. 
The upper part fgrs of the ftage is to be raifed or low¬ 
ered occafionally, in order to make the centre of the ob¬ 
ject coincide with the centre of the lens at O. 
To delineate objects, the grey glafs muft be placed be¬ 
fore the large lenfes ; the pifture of the objeft will be 
formed on this glafs, and the outline may be accurately 
taken by going over the pifture with a pencil. 
The opaque part may be ufed in the day-time, without 
a lamp, provided the large lenfes at AB are fcreened from 
the light. . . ... 
To ufe the lucernal microfcope in the examination ot 
tranfparent objeBs. The inftrument is to remain as before : 
the upper part fgs of the opaque ftage muft be removed, 
and the ftage for tranfparent objefts, reprefented at fig. 10, 
put in its place j the end 9, 10, to be next the lamp. 
Place the greyed glafs in its groove at the end AB, and 
the objefts in the fiider-holder at the front of the ftage 5 
then tranfmit as ftrong a light as you are able on the ob¬ 
ject, which you will eafily do by railing or lowering the 
lamp. The objeft will be beautifully depifted on the 
grey glafs: it muft be regulated to the focus of the mag¬ 
nifier, by turning the pinion a. The objeft may be 
viewed either with or without the guide for the eye. A 
fingle obferver will fee an objeft to the greateft advantage 
by ufing this guide, which is to be adjufted as we have 
defcribed above. If two or three wilh to examine the ob¬ 
jeft at the fame time, the guide for the eye mult be laid 
afide. 
Take the large lens out of the groove, and receive the 
image on the grey glafs; in this cafe, the guide for the 
eye is of no ufe : if the grey glafs be taken away, the 
image of the objeft may be received on a paper fcreen. 
Take out the grey glafs, replace the large lenfes, and ufe 
the guide for the eye ; attend to the foregoing directions, 
and adjuft the objeft to its proper focus. You will then 
fee the objeft in a blaze of light almoft too great for the 
eye ; a circumltance that will be found very ufeful in the 
examination of particular objefts. The edges of the.ob- 
jeft in this mode will be fomewliat coloured; but, as it is 
only ufed in this full light for occafional purpofes, it has 
been thought better to leave this fmall imperfeftion, than, 
by remedying it, tofacrifice greater advantages; the more 
fo, as this fault is eafily correfted, and a new and intereft- 
ing view of the objeft is obtained, by turning the inftru¬ 
ment out of the direft rays of light, and permitting them 
to pafs through only in an oblique direftion, by which 
the upper furface is in fome degree illuminated, and the 
objeft is feen partly as opaque, partly as tranfparent. It 
has been already obferved, that the tranfparent objefts 
might be placed between the Aider-holders of the ftage 
for opaque objefts, and then be examined as if opaque. 
Some tranfparent objefts appear to the greateft advantage 
when the lens at 9, 10, is taken away ; as, by giving too 
great a quantity of light, it renders the edges lefs ftiarp. 
The variety of views which may be taken of every objeft, 
by means of the improved lucernal microfcope, will be 
found to be of great ufe to an accurate obferver: it will 
give him an opportunity of correfting or confirming his 
difcoveries, and inveftigating thofe parts in one mode 
which are invifible in another. 
To throw the image of tranfparent objects on a fcreen, as 
in the folar microfcope.— It has been long a_ micro.fcopical 
defideratum, to have an inftrument by which the image 
of tranfparent objefts might be thrown on a fcreen, as in 
the common folar microfcope 5 and this, not only becaufe 
the fun is fo uncertain in this climate, and the ufe of the 
I C S. (528 
folar microfcope requires confinement in the fineft part of 
the day, when time feldom hangs heavy on the mind; but 
as it alfo affords an increale of pleafure, by difplaying its 
wonders to feveral perfons at the fame inftant, without the 
leaft fatigue to the eye. This purpofe is now eft’eftually 
anfwered, by affixing the tranfparent ftage of the lucernal 
to a lantern, with one of Argand’s lamps. Tlifc lamp 
is placed within the lantern ; and the end 9, 10, of the 
tranfparent ftage is fcrewed into a female fcrew, which is 
rivetted in the Aiding part of the front of the lantern; 
the magnifying lenfes are to be fcrewed into the hole re¬ 
prefented at fig. 12. and they are adjufted by turning the 
milled nut. The quantity of light is to be regulated by 
raifing and lowering the Aiding-plate or the lamp. 
The apparatus which ufually accompanies the improved 
lucernal microfcope is—The ftage for opaque objefts, with 
its f’emicircular lump of glafs, and concave mirror. The 
ftage for tranfparent objefts, which fits on the upper part 
of the foregoing ftage. The ftiding-tube, to which the 
magnifiers are to be affixed: one end of thefe is to be 
fcrewed on the end D of the wooden body ; the magnifier 
in ufe is to be fcrewed to the other end of the inner tube. 
There are eight magnifying lenfes, fo conftrufted that 
they may be combined together, and thus produce a very 
great variety of magnifying powers. A fifii-pan, fuch as 
is reprefented at Plate XI. fig. 6. A fteel wire, with a 
pair of nippers at one end, and a fmall cylinder of ivory 
at the other; Plate XI. fig. 5. A Aider of brafs, con¬ 
taining a fiat glafs Aider, and a brafs Aider, into which are 
fitted fome fmall concave glaffes. A pair of forceps. Six 
large and fix fmall ivory Aiders, with tranfparent objefts. 
Fourteen wooden Aiders, with four opaque objefts in each 
Aider ; and two fpare Aiders. Some capillary tubes for 
viewing fmall animalcula. One of the improved Argand 
lamps, which are the molt fuitable for microfcopic pur¬ 
pofes, either with this or any other inftrument, on ac¬ 
count of the clearnefs, intenfity, and fteadinefs, of the 
light. 
To calculate the Magnifying Powers of Microfcopes. 
If the focus of a convex lens, e. g. of a fingle ?nicro~ 
fcope, be at one inch, and the natural fight at eight inches, 
which is the common llandard, an objeft may be feen 
through that lens at one inch diftance from the eye; and 
will appear, in its diameter, eight times larger than to 
the naked eye. But, as the object is magnified every way 
equally, in length as well as breadth, we muft fquare this 
diameter, to know really how much it appears enlarged; 
and we ffiall then find, that its fuperficies is indeed mag¬ 
nified fixty-four times. 
Again: Suppofe a convex lens whofe focus is at one- 
tenth of an inch diftant from its centre ; in eight inches 
there are eighty fuch tenths of an inch : and therefore, an 
objeft may be feen through this lens eighty times nearer 
than it can diftinftly by the naked eye. It will confe- 
quently appear eighty times longer, and eighty times 
broader, than it does to common fight; and, as eighty 
multiplied by eighty makes 6400, fo many times it really 
appears magnified. 
To go one ftep farther: If a convex glafs be fo fmall, 
that its focus is no more than one-twentieth of an inch 
diftant, we ffiall find that eight Undies, the common dif¬ 
tance of fight, contains 160 of thefe twentieth parts; 
and, in confequence, the length and breadth of an ob¬ 
jeft, when feen through fuch lens, will each be magnified 
160 times ; which, multiplied by 160, to give the fquare, 
will amount to 25,600 : and fo many times, it is plain, 
the fuperficies of the objeft muft appear larger than it 
does to the naked eye at the diftance of eight inches. 
Therefore, in a fingle microfcope, to learn the magni¬ 
fying power of any glafs, no more is neceffary than to 
bring it to its true focus ; the exaft place of which will 
be known, by an objeft appearing perfeftly diftinft and 
ffiarp when placed there. Then, with a pair of fmall 
compaffes, meafure, as nearly as you can, the diftance 
from 
