Mr. herschel on the Georgium Sidus. 5 
have 4" 36 for the diameter of the planet. On comparing 
the menfures then with this mean, we find hut two of them 
that differ fomewhat more than half a fecond from it ; the reft 
are almoft all within a quarter of a fecond of that meafure. 
This agreement, in the dimensions of any other planet, would 
appear very conliderable ; but not being fatisfied, when I 
thought it poflible to obtain much more accurate meafures, I 
employed the lamp -micrometer in preference to the former. 
The firft time I ufed it upon this occalion I perceived, that if, 
inftead of two lucid points, we could have an intire lucid dilk to 
refemble the Planet, the meafures would certainly be ftill more 
compleat. The difficulty of dilating and contracting a figure 
that fhould always remain a circle, appeared to me very confi- 
derable, though nature, with her ufual fimplicity, holds out to 
us a pattern in the Iris of the eye, which, fimple as it appears, 
is not one of the leaft admirable of her inimitable works. 
However, I recollected, that it was not abfolutcly requi- 
fite to have every infenlible degree of magnitude ; lince, by 
changing the diftance, I could without much inconvenience 
make every little intermediate gradation between a let of circles 
of a proper fize, that might be prepared for the purpofe. 
Intending to put this defign into practice, I contrived the fol- 
lowing apparatus. 
A large lanthorn, of the conftruCtion of thofe fmall ones 
that are ufed with my lamp-micrometer*, muft have a place for 
three flames in the middle, which is neceflary, in order that 
we may have the quantity of light required, by lighting one, 
two, or all of them. The grooves, inftead of brals Aiding 
doors, muft be wide enough to admit a pa fte- board, and three 
or four thickneffes of paper. I prepared a let of circles, cut 
* Phi!. Tranf. vol. LXXIT. p. 166. 
cut 
