( 125 ) 
fhould be of the diftance given , having the Focus of 
the Objeft-glafi given equal to B, and the diftance 
given to B + Q * the diftance between the firft and the 
fecond Glafs will be equal to whence fubdu&ing B 
(the Focus of the Qbjeft-glafs given) there remains ??- • 
• zB-jD j 
and if this fum be fuppofed equal to C, we (hall eafily 
know, by the precedent Rule ,the Focus of th efecond Glaft. 
So far M. Auzout, who, I truft, will receive due fatisfa « 
&ion to his defire, as foon as the happy end of the prefen t 
Contagion fhall give a beginning and life again to the Studies 
and Aftions of our retired Philofophers , 
I (hall onely here adde, That the Secret he mentions [of 
weafuring the diftance of Places by a Telefcope {fitted for that 
purpofe) andfromoneftation ] is a thing already known (if 
I am not misinformed) to fome Members of our Society 5 
who have been a good while fince confidering of it, and 
have contrived ways for the doing of it .* Whether the lame 
with thofe of Mr. Au%oui , I know not. Nor have I (at the 
diftance that I am now from them ) opportunity of particu- 
lar Information. 
An Experiment of a way of preparing a Liquor-, that 
fhall finh^into, and colour the whole Body of Mar- 
ble, caufmg a Pi&ure, drawn on a furface 5 to 
appear alfo in the inmoft parts of the Stone . 
This Experiment^ having been hinted at in the next fore- 
going Papers , out of the Mundus Subterraneus of Athanafim 
Kircher , and feveral Curious Perlbns, who either have not 
the leifure to read Voluminous Authors, or are not readily 
skilled in that Learned Tongue wherein the laid Book is 
written, being very defirous to have it transferred hither, 
it was thought fit to comply with their defire herein. 
The Author therefore of the Mundus , See, having feen 
fome 
