518 » Mr. herschel’s AJlronomlcal Qbfervations 
opportunity of particularly obferving the three moun- 
tains mentioned by hevelius; nor that which riccio- 
lus found to project a sixteenth part of the Moon’s dia- 
meter. If keill had calculated the height of this laft 
mentioned hill according to the theorem I have given, 
he would have found (fuppofing the obfervation to have 
been made, as he fays, on the fourth day after new 
Moon) that its perpendicular could not well be lefs than 
between eleven and twelve miles. 
I {hall not fail to take the firlt opportunity of obferv- 
ing thefe four, and every other mountain of any emi- 
nence ; and if other perfons, who are furnifhed with 
good telefcopes and micrometers, would take the quan- 
tity of the projection of the lunar mountains, I make no 
doubt, but that we fhould be nearly as well acquainted 
with their heights as we are with the elevation of our 
own. One caution I would beg leave to mention to thofe 
who may ufe the excellent 3^ feet refractors of Mr. 
DOLLOND. The admirable quantity of light, which on 
molt occalions is fo delirable, will probably give the mea- 
fure of the projection fomewhat larger than the true, if 
not guarded againft by proper limitations placed before 
the objeCt glafs. I have taken no notice of any allow- 
ance to be made for the refraction a ray of light muft 
fuffer in palling through the atmofphere of the Moon* 
•when 
