wn, the distance of them will be known. If it be supposed,
that the _Focus_ of the _first_ be B. and _that_ of the _second_ C, and the
distance given, B + 2D, and that D _minus_ C, be _equal_ to F; for, this
distance will be _equal_ to B + C + F - rF squared - C squared. And if you have the
_Focus_ of the _first Object-glass_, equal to B, the distance, where you
will put the _second_ Glass equal to B + C + D, the _focus_ of the 2d
Glasse will be found equal to CD/{C+D}. And if you will that the Object
shall be magnified as much with these two Glasses, as it would be with a
single one, whereof the _Focus_ {125} should be of the distance given,
having the _Focus_ of the _Object-glass_ given equal to B, and the distance
to B + D; the distance between the first and the second Glass will be equal
to {2B squared + 2BD}/{2B + D}, whence subducting B (the _Focus_ of the
_Object-glass_ given) there remains BD/{2B + D}; and if this sum be
supposed equal to C, we shall easily know, by the preceding Rule, the
_Focus_ of the _second_ Glass.

So far M. _Auzout_, who, I trust, will receive due satisfaction to his
desire, as soon as the happy end of the present Contagion shall give a
beginning and life again to the Studies and Actions of our retired
_Philosophers_.

I shall onely here adde, That the Secret he mentions [_Of measuring the
distance of Places by a Telescope (fitted for that purpose) and from one
Station_] is a thing already known (if I am not mis-informed) to some
Members of our Society; who have been a good while since considering of it,
and have contrived ways for the doing of it: Whether the same with those of
Mr. _Auzout_, I know not. Nor have I (at the distance that I am now from
them) opportunity of particular Information.

* * * * *

_An Experiment of a way of preparing a Liquor, that shall sink into, and
colour the whole Body of _Marble_, causing a _Picture_, drawn on a surface,
to appear also in the _inmost_ parts of the Stone._

This _Experiment_, having been hinted at in the next foregoing _Papers_,
out of the _Mundus Subterraneus_ of _Athanasius Kircher_, and several
Curious Persons, who either have not the leisure to read Voluminous
Authors, or are not readily skilled in that Learned Tongue wherein the said
Book is written, being very desirous to have it transferred hither, it was
thought fit to comply with their desire herein.

The Author therefore of the _Mundus_, &c, havi