f 101 3 
adlel to the back-horizon-glafs ; when this ad-- 
juftment may be performed in the fame manner as 
the correfponding adjuflment of the fore-horizon- 
glafs. But the bending of the index, that follows 
the fetting it off the arch, is a very difagreeable cir-. 
cumltance, having a tendency, efpecially on board of 
fhip, to expofe both the index and centre work to, 
damage; and may even, without extraordinary pre- 
cautions taken by the inftrument maker in placing 
the plane of the index-glafs exactly according to the 
length of the index, difturb its perpendicularity to 
the plane of the quadrant : on thefe accounts it would 
be much better if this adjuftment of the back-horizon- 
glafs could be performed, like thofe of the fore-, 
horizon-glafs, with the index remaining upon the 
arch of the quadrant. Fortunately, this dejideratlim . 
has been lately effeffed by an ingenious contrivance 
invented by Mr.. Dollond, which he has given an 
account of in a letter addreffed to me'*, which I have 
prefented to this Society, by means of an additional 
index applied to the back-horizon glafs ; whereby 
both the adjuftments may be made by the fame ob- 
fervatians and with nearly the fame exadlnefs as thofe. 
of the fore-horizon*glafs : for a rarther knowledge off 
which, fee the account itfelf. 
Befides the difficulty of adjuffing the back- horn* . 
zon-glafs, the want of a method of directing the 
line of fight parallel to the plane of the quadrant, 
has proved alfo a eonfiderable obftacle to the ule 
of the. back-obfervation this will eafiiy appear 
from the following proportion, that the error of. 
the angle meafured arifing from any fma.ll de- 
* Seethe XIVth paper,. .which, immediately precedes this.. 
vj&tibft] 
