OPT 
lated, is confirmed, the archbifhop of the province, by a 
cuftomary p rerogative, claims the collation of the fir vacant 
beasties or dignity, in that fee, according as he fhall choofe ; 
which choice 1s called the archbifhop’s opticn. 
Ever = archbifhop Cranmer’s time, the mode hath 
been to convey the advowfon, either of the firft dignity or 
benefice that fhould fall, or of fome one certain, to the arch- 
ifhop, his executors, or oo. at firft for 21 years, and 
afterwards for the next avoidanc 
But in cafe the bifhop dies or is tranflated, before the 
motion chofen by the arch- 
that archbifhop dies 
before the avoidance fhall happen, the right of Sra up the 
vacancy fhall go to his executors or adminiftrato 
OPTO- vie in rhe ape a town sot Chinefe 
Tartary. N. lat. 4 E. long. 92° 
OPTO OMETER, an le Cee by Dr. Porter- 
field, and fo named by him rom its ufe i in meafuring the 
Let an ob 
fingle point. 
» or removed Gackeacda. to I K, the {mall ee 
paffing through the perforations, will no longer meet i 
fingle point, but will fall on two diftin& {pots of the clsnes 
»H,1I,K 3 and, in either cafe, form a double image of 
j © more radiating points, 
when the plane, which 
receives the images, pafles through the focus of rays coming 
from the fir 
point, and within that of rays coming from the neareft. 
Upon this Lap ae Dr. Porterfield’s optometer was 
founded. t if the three points be fuppofed to be as 
R (fg. 3-)s 
OPT 
2 fingle ; but in every other cafe, the image being double, 
e fhall appear to fee a double obje& : and if we look at a 
fine a nearly to the eye, it will appear as two lines, 
cro each other in the point of perfect vifion. For this 
pare e, the holes may be converted into flits, which render 
the images nearly as diftin@, at the fame time that ad 
admit more light. The number may be increafed from t 
to four, or more, whenever particular inveftigations on 
it neceffz 
The op tometer may be made of a flip of card-paper, or 
of ivory, about eight inches in length, and one in breadth, 
sedi longitudinal by a black line, which muft n t be 
00 yas fe of the card muft be cut, as is tea 
fy 4s it may be turned up, and fixed in an in- 
clined peices iy means of the fhoulders; or a detached 
piece, nearly o din orm, may be ed tas to the isda il 
it is here ved. =e Je about half an 
t t he efbyopic eyes, the 
other end muft be furnifhed with a lens of four inches focal 
according to the table annexe 
ae diverging, but alfo parallel ae converging rays from 
the lens are referr ed to their saa ocus. The in hy 
m 
calculate 
rom the truth. 
che firft trial, will fx 
eee oo ant yen which co fuits the defee ce) 
ew can bring their eyes at pleafure to the ftate of 
full pre or per perfect relaxation ; and a power 
as will point out the fpeétacles 
neceffary for a prefbyopic eye, to fee at 12 and at 18 inches 
refpectively: the m 
rument | ould be fcreened from * view. 
interfeCtion may be accurately afcertained, by means of an 
index fliding along the fcale. e optometer is ceoeeleated 
in figs. § and 6; and the manner in which the lines appear 
in “fig. 7 
TARLE 
