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should be of the distance given, having the Focus of the Object-glass given equal to $\mathrm {B}$, and the distance given to $\mathrm {B} +\mathrm {D} ^{2}$; the distance between the first and the second Glass will be equal to $\scriptstyle {\tfrac {2B^{2}+2BD}{2B+D}}$, whence subducting $\mathrm {B}$ (the Focus of the Object-glass given) there remains $\scriptstyle {\tfrac {2BD}{2B+D}}$; and if this sum be supposed equal to $\mathrm {C}$, we shall easily know, by the precedent 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 knew 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 on 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, having seen