C *74 ] 
v no of the two fruftums muft be brought, after the 
mirror is diametrically bifecfte i. 
Let the concave fide be defended, by pading a 
paper all over it, and then let it be divided with a 
faw in the diameter d c m ; taking care that the faid 
diameter be in the middle of the kerf, which may 
be as broad as the fpace between the lines a 0, e q. 
Let the afperities of the edges of both fruftums be 
ground off, that they may be very ft r ait after their 
being fawn. 
Fig ' 7 • 
Reprefents a thick round plate of brafs very plain, 
and equally thick all over, having lines drawn on it, 
as on Fig. 2. alfo one line on each lide of the diame- 
ter dm , equidiftant from it, and parallel thereto. 
The diftance of thefe two lines a 0, e y, from one 
another equal to the kerf of the faw, which divided 
the mirror. The diameter of this plate muft be 
equal to that of the mirror before it was divided 
On the under fide of the plate muff be two pins 
fattened thereto, 1 1 , their diameters equal to the kerf 
of the faw, that they may keep the two frudums 
of the mirror at the fame dittance from each other 
that they were before their divifion ; fo ttaall their 
circular edges be extended as far as the circumfe- 
rence of the plate, and their ttrait edges touch the 
faid pins in the lines ao, e q. 
The end of the tube muft be turn’d on the infide 
exadfly to fit the plate and mirror, that they may 
not Aide any way, for that would fpoil the obferva- 
tions. 
In the diameter of the plate r s, on the points v, w, 
diftant half an inch from c, the center of the plate, 
and 
