630 
OPT 
contrivances for adapting it to microfcopical obfer- 
vations. Leeuwenhoek’s method of eftimating the fize 
of fmal! objeCts was by comparing them with grains of 
fand, of which 100 in a line took up an inch. Tliefe 
grains he laid upon the fame plate with his obje&s, and 
viewed them at the fame time. Dr. Jurin’s method was 
fimilar to this; for he found the diameter of a piece of 
fine filver wire, by wrapping it as clofe as he could about 
a pin, and obferving how many rings made an inch ; and 
he ufed this wire in the fame manner as Leeuwenhoek 
employed his fand. Dr. Hooke looked upon the magni¬ 
fied objeftwith one eye, while at the lame time he viewed 
other objefls placed at the fame diftance with the other 
eye. In this manner he was able, by the help of a ruler, 
divided into inches and (mall parts, and laid on the pe- 
deftal of the microfcope, to caff as it were the magnified 
appearance of the objeCt upon the ruler, and thus exactly 
to meafure the diameter which it appeared to have through 
the glafs; which, being compared with the diameter, as 
it appeared to the naked eye, flowed the degree in which 
it was magnified. A little practice, fays Mr. Baker, will 
render this method exceedingly eafy and pleafant. 
Mr. Martin, in his Optics, recommends fuch a micro¬ 
meter for a microfcope as had been applied to telefcopes; 
for he advifes to draw a number of parallel lines on a 
piece of glafs with the fine point of a diamond, at the dif¬ 
tance of the 40th of an inch from one another, and to 
place it in the focus of the eye-glafs. By this method, 
Dr. Smith contrived to take the exaCt draught of objects 
viewed by a double microfcope; for he advifes to get a 
lattice, made with fmall filver wires or fquares, drawn 
upon a plain glafs by the ftrokes of a diamond, and to put 
it into the place of the image formed by the objedt-glafs. 
Then, by transferring the parts of the objeCt feen in the 
fquares of the glafs or lattice upon fimilar correfponding 
fquares drawn on paper, the picture may be exactly taken. 
Mr. Martin alfo introduced into compound microfcopes 
another micrometer, confifting of a fcrew. See both thefe 
methods defcribed in his Optics, p. 277. 
The mode of aCtual admeafurement, as Mr. Adams ob- 
ferves, (Microfcopical ElTays, p. 57.) is, wdthout doubt, 
the mod fimple that can be ufed ; as by it we comprehend, 
in a manner, at one glance, the different efteCts of com¬ 
bined glaffes; and as it faves the trouble, and avoids the 
obfcurity, of the ufual modes of. calculation: but many 
perfons find it exceedingly difficult to adopt this method, 
becaufe they have not been accuftomed to obferve with 
both eyes at once. To obviate this inconvenience, the 
late Mr. Adams contrived an inftrument called the nceclle- 
vuicromctcr, which was firff defcribed in his Micrographia 
Illuftrata ; and of which, as now conftru&ed, we have 
the following defcription by his fon, Mr. George Adams, 
in the ingenious Eflays above quoted. 
This micrometer confifts of a fcrew, wdiich has 50 
threads to an inch ; this fcrew carries an index, which 
points to the divifions on a circular plate, which is fixed 
at right angles to the axis of the fcrew. The revolutions 
of the fcrew are counted on a fcale, which is an inch di¬ 
vided into 50 parts; the index to thefe divifions is a 
flower-de-luce marked upon the'Aider, which carries the 
needle-point acrofs the field of the microfcope. Every 
revolution of the micrometer-fcrew meafures the 50th 
part of an inch, which is again fubdivided by means of 
tile divifions on the circular plate, as this is divided into 
20 equal parts, over which the index pafles at every re¬ 
volution of the fcrew; by which means we obtain with 
eafe the meafure of 1 oooth part of an inch : for 50, the 
number of threads on the fcrew in one inch, being mul¬ 
tiplied by 20, the divifions on the circular plate, are equal 
to 1000 ; fo that each divifion on the circular plate fhows 
that the needle has either advanced or receded 1000th 
part of an inch. To place this micrometer on the body 
of the microfcope, open the circular part FKH, fig. 12. 
by taking out the fcrew G ; throw back the femicircle 
FK, which moves upon a joint at K; then turn the Aiding 
I c s. 
tube of the body of the microfcope, that the fmall holes 
which are in both tubes may exaCtly coincide, and let 
the' needle g• of the micrometer have a free paffage through 
them ; after this, fcrew it faff upon the -body by the 
fcrew G. The needle will now traverfe the field of the 
microfcope, and meafure the length and breadth of the 
image of any object that is applied to it. But further 
affiftance nuift be had, in order to meafure the objedt 
itfelf, which is a fubjeCt of real importance; for, though 
we have afcertained the power of the microfcope, and 
know that it is fo many thoufand times, yet this will be 
of little affiftance towards afcertaining an accurate idea of 
its real fize ; for our ideas of bulk being formed by the 
comparifon of one object with another, we can only judge 
of that of any particular body, by comparing it with an¬ 
other whofe fize is known : the fame thing is neceflary, 
in order to form an eftimate by the microfcope ; therefore, 
to afcertain the real meafure of the objeCt, we muff make 
the point of the needle pafs over the image of a known 
part of an inch placed on the ftage, and write down the 
revolutions made by the fcrew w'hile the needle pafled 
over the image of this known meafure ; by which means 
we afcertain the number of revolutions on the fcrew 
which are adequate to a real and known meafure on the 
ftage. As it requires an attentive eye to watch the mo¬ 
tion of the needle-point as it pafles over the irpage of a 
known part of an inch on the ftage, we ought not to truft 
to one fingle meafurement of the image, but fhould 
repeat it at leaft fix times ; then add the fix meafures thus 
obtained together, and divide their fum by fix, or the 
number of trials; the quotient will be the mean of all 
the trials. -This refult is to be placed in a column of a 
table next to that which contains the number of the mag¬ 
nifiers. 
By the affiftance of the feftoral fcale, we obtain with 
eafe a fmall part of an inch. This fcale is fhown at figs. 
13, 14, 1 5. in which the two lines ca,cb, with thefides ab, 
form an ifofceles triangle ; each of the tides is two inches 
long, and the bafe ftill only of one-tenth of an inch. The 
longer iides may be .of any given length, and the bafe ftill 
only of one-tenth of an inch. The longer lines may be 
confidered as the line of lines upon a feCtor opened to 
one-tenth of an inch. Hence, whatever number of equal 
parts ca, cb, are divided into, their tranfverfe meafure 
will be fuch a part of one-tenth as is exprefl'ed by their 
divifions. Thus, if it be divided into ten equal parts, this 
will divide the inch into 100 equal parts ; the firft divifion 
nexte will be equal to 100th part of an inch, becaufe it is 
the tenth part of one-tenth of an inch. If thefe lines are 
divided into twenty equal parts, the inch will be by that 
means divided into 200 equal parts. Laftly, if ab, ca, are 
made three inches long, and divided into 100 equal parts, 
we obtain with eafe the 1000th part. The fcale is repre- 
fented as folid at fig. 13. but as perforated at fig. 14 and 
15. fo that the light pafles through the aperture, w r hen the 
feftoral part is placed on the ftage. 
To ufe this fcale, firft fix the micrometer, fig. 12, to the 
body of the microfcope ; then fit the fedtoral fcale, fig. 15, 
in the ftage, and adjuft the microfcope to its proper focus 
or diftance from the fcale, which is to be moved till the 
bafe appears in the middle of the field of view ; then bring 
the needle-point g’, fig. 12, by turning the fcrew L, to 
touch one of the lines ca, exaCtly at the point anfwering 
to 20 on the feCtoral fcale. The index a of the micro¬ 
meter is to be fet to the firft divifion, and that on the 
dial-plate to 20, which is both the beginning and end of 
its divifions; we are then prepared to find the magnifying- 
power of every magnifier in the compound microfcope 
which we are ufing. 
Example. Everything being prepared agreeable to the 
foregoing directions, fuppofe you are defirous of afcer¬ 
taining the magnifying-power of the lens N° 4. turn the 
micrometer-fcrew until the point of the needle has pafled. 
over the magnified image of the tenth part of one inch ; 
then the divifion, where the two indices remain, will fliow 
how 
