OBJ 
There are feveral conditions requifite to an object of fenfe 3 
as, that it be material; that it be within a certain diftance 
of a competent extent; its fenfible os fufficiently in- 
pee 
texture of their furface, difpofing them to reflect differently 
coloured rays, occafions in us feveral fenfations of colour, 
which we attribute to them 
the hole of a darkened roo y this 
land{cape = ad objects abroad, painted peed on the 
back of t —How, inthis cafe, the objets which are 
painted inverted fhould be feen He is matter of contro- 
ve AMERA Obfcura, and Vision. 
Oxss ey of a telefcope or microfcope is the glafs 
paced a se end of the tube Sicha is — the objeG. 
ya pistes arity and hese es an objed glafs, 
c circles on paper. one having its 
with the breadth of ‘the ebiee gtle ; the 
other half that eee divide the inner a priate into 
glafs ; if they do, we may be affured of ie regularity af 
the glafs, that is, of rm; and, at me time, 
we obtain exadtly the glafs’s focal a th. 
pie is fearcely any better way of proving the 
objet be, 
-glafs, than by placing i it. in a tu 
al {mall eye-glaffes, at feveral diftan t ob- 
jeGts ; for that objeGt-glafs which reprefents objects the 
heaieet and moft diftinét, which bears the greateft aperture, 
and moft convex and concave eye-glafs, without a 
or hazinefs, is always the beft. 
A circular object-glafs is faid to be truly centered, when 
the centre of its circumference is fituated in the axis “of the 
glafs; and to be ill-centered, when the centre of its circum- 
Top sees befide the axis. 
ve whether objed-glaffes be Beige ect hold 
the ae as due diftance from the eye, obferve the 
two reflected images of a poses. : ier cee images unite, 
or coalefce, there is the true centre. this be in t 
middle, or central point of the glafs, then it is known to be 
abe centered, 
e are various methods of finding the ed centre of an 
g was the metho 
Indeed, 
excellency of a 
g it 
oO 
narrower be fo exactly fitted to the concavity of the wider, 
as juft to turn round in it with eale, but without waddling ; 
and let the planes of the bafes of the tubes be exadily per- 
i ae to their fides. Place the bafe of the narrower 
n a {mooth brafe plate, or a wooden board, of an 
ail ‘thicknchs =~ with any fharp-pointed tool, defcribe 
true circle upon the board round the outward circum- 
ference of the bafe ; et upon the centre of this circle, to 
OBJ 
be found hoe the tube is removed, defcribe a larger circle 
upon the 
upon its axis; and if 
in this axis, the pidiure will be 
{creen ; if not, ever oint of 
e€ t 
pofition, ae the objeét-glafs till the point aforefaid falls 
exadtly in the middle between the two marks. Then turn 
the tube noun again, and the point of the aad will either 
reft there, or will defcribe a much {maller circle than before, 
its centre of refraction, a l late acb 
be bent fquare at e , as reprefented in the figure 
leaving a piece in the aie tralian in length to the dia- 
is aefeibing it may xed firmer with wax, or harder 
cem = Gptics, book iii. chap 
gives the following rule, hich i is 
very ¢ 
r 
ed by ham 
a “Tittle wider than, the next, and nu &c. in 
their proper order, each of them ig yi at ee eo 
than at the entrance. (Plate » jig. 2 
one of thefe notches to the ncket aac of ile Tice dalien 
fo that the edge may reach to about half its depth; and if 
the oppofite fide pafs to the bottom of the notch, grind the 
lens narrower on the thinneft fide, till you find it at that a 
