f '68] 
I have therefore contrived fome dioptric telefcopes, 
and a reflecting one j either of which (by reprefent- 
ing the objeCt double) will, if well made, anfwer the 
deflgn. 
Fig. i. 
Reprefents the whole body of the fun, as it appears 
double, and magnified in the telefcope. Let an be the 
diameter of the one, and rx of the other image of the 
fun inperigao ; fo fhall nr be the diftance between the 
two images at that time; which meafured with the 
micrometer is equal to (fuppofe) io feconds. 
Let bm be the diameter of the one folar image, and 
sw of the other, when in apogreo : fo fhall ms be the 
then diflance of the folar images, mcafuring with the 
micrometer (fuppofe) i minute io feconds. The dif- 
ference of thefe two obfervations, i minute, is the 
apparent diminution of the fun’s diameter. 
The little circle, whofe diameter is dt , is the whole 
area vifible at once in the telefcope, which is not one 
third part of the magnified diameter of the fun : but 
lince both nr at one time, and ms at another time, 
are vilible within the telefcope’s area, (if good inflxu- 
ments are procured) I can fee no difficulty in perform- 
ing what I have propofed above more accurately than 
it has ever yet been done, except this one (which 
fome time, fince Mr. Graham in a letter to me men- 
tion’d) viz. that of defining the fun’s difk truly; and 
I think to do that to good perfection, is beyond hu- 
man art. A telefcope for this ufe may be made to 
magnify the fun’s diameter to any degree whatfoever, 
not exceeding fuch degree, as will make any part of 
the line m s fall without the area of the telefcope : 
and I think it will be very difficult to make one with 
a charge 
