[ S3 ] 
point within the objedt-glafs about one third of its 
thicknefs from that furface which is towards the 
wires, if the glafs be, as ufual, equally convex on 
both Tides; but if the focus of the objedt-glafs 
is pretty long and its thicknefs not great, the error 
that can arile by meafuring from any part of the 
objedt-glafs will become infenfible as to the altera- 
tion in the angle. 
The table for the micrometer may likewife be 
made by fetting up two marks at a diftance on the 
ground, and obferving with the micrometer the 
revolutions, 8cc. which they fubtend when feen 
through the telefcope, and then computing the 
angles thofe objects iubtend at the objedt -glafs, by 
meafuring their diftance from each other and from 
the objedt-glafs. The like may alfo be done by 
opening the threads to any number of revolutions, 
and then making a ftar move exadtly upon the per- 
pendicular thread, and noting the time it is pafftng 
from one parallel thread to the other ; for that time 
turned into minutes and feconds of a degree, by 
allowing for the ftar’s declination and going of the 
clock, &c. will be the angle anfwering to the num- 
ber of revolutions ; from which the whole table 
may be made. This method perhaps might be 
moft advantageoufly pradtifed in ftars near the pole, 
Where the apparent motion being flow a fecond in 
time will anfwer to a much fmaller angle than 
towards the equator. But 1 believe, upon trial, 
the firft method will be found moft eafy and prac- 
ticable, efpecially if the fcale made ufe of be well 
divided. 
VII. A 
