52 a Mr. hers-chel’s AJlronomical Qbfervatims 
'The following additional Memoranda of the Manner in 
which Mr. herschel made his Ohfervations are taken 
from a Letter of his to the rev. Dr. maskelyne, Jflro- 
nomer Royal. 
I ' - 
I N the fecond figure of my ohfervations, the points 
l, s', e, r, are all fuppofed to he in one plane; and as the 
illuminating ray sl is alfo in this plane, it follows, that 
the line l n on) will always be perpendicular to the 
right line which joins the cufps of the Moon ^ ; and the 
truth of the theorem there delivered depends upon this 
circumftance. 
For this reafon I have taken care in all my obferva- 
tions to meafure the line, which in fig. 3. (taken from 
your letter to Dr. watson) is marked on, parallel to the 
line cd, or perpendicular to ab, and not the line r«, per- 
pendicular to the elliptical curve Arcs. 
The manner of taking it is eafy enough : however, I 
h ave occafionally ufed three different methods, and. will de- 
(c) It is here fuppofed, that rays from the Sun s, and the eye of the obferver 
x, to any part of the Moon L, may be taken for parallel ; and therefore, that 
different planes, made by feveral feclions of the Moon, according as the 
point l is taken North or South of the diameter of the Moon, which is at 
re&angles to the line joining the cufps, may alfo be taken to be parallel to that 
•diameter. 
fcribe 
