To diminish some of the afforesaid disadvantages there may be other still new varia
tions or additions to these designes. As for instance by using two eye-glasses. Suppose
$\mathrm{CD}$ represent the great Concave, $\mathrm{F}$ the little speculum
$\mathrm{E}$ the eye-glass & $\mathrm{G}$ another double-convex glass
between $\mathrm{E}$ & $\mathrm{F}$ on both sides of w^ch the rays cross.
This way of redoubling these Tubes seems not
inferior to the rest. For thus the object appeares
erect, the speculum $\mathrm{F}$ intercepts less light, & the charge
may be varied at pleasure onely by changing the positions of $\mathrm{G}$ & $\mathrm{F}$. But
yet this is not w^th out its imperfections, & particularly the [illeg] (besides those common
w^th the other Designes) the glass $\mathrm{G}$ will intercept many of the best rays in
their passage from the concave $\mathrm{CD}$ to y^e little speculum $\mathrm{F}$, unless it be
made less then is consistent w^th some other conveniences. And by the iterated
decussations of the rays objects will be rendered less distinct, as is manifest in
Dioptrick Telescopes where two or three eye-glasses are applyed to erect the
object.

As to the attempt in w^ch M^r Rieve was imployed, I presumed it had been done w^th
much more accurateness then M^r Gregory now signifys, because M^r Hook, who
you know is a curious & accurate experimenter, affirms in his considerations
on my letter to M^r Oldenburg concerning refractions & colours published in the
Transactions Numb 80, that he made severall experiments w^th that Instrument.
And though he lays the blame on M^r Rieve's encheiria, yet he says not y^t
he blamed him then when the Experiment was made. His words are these

" I have made many tryalls both for Telescopes & Microscopes by reflexion w^ch I
have mentioned in my Micrographia, but deserted it as to Telescopes when I con-
sidered that the focus of a sphericall concave is not a point but a line, & that
the rays are less true reflected to a point by a Concave then refracted by a
Convex, w^ch made me seek that by refraction w^ch I found could not be ex-
pected from reflexion. Nor indeed could I find any effect of it by one of six
foot radius w^ch about 7 or 8 yeares since M^r Rieve made for M^r Gregory
w^th w^ch I made severall tryalls; but it now appeares that it was for want
of a good encheiria; From w^ch cause many good experiments have been lost. Both
w^ch considerations discouraged me from attempting further that way; especially
since I found the Parabola much more difficult to describe then the Hyperbola or
Ellipsis.

From hence I might well infer that y^e want of a good encheiria appeared
not till now. And that M^r Hook was discouraged from attempting further that way
onely by these two \or three/ considerations, That a Convex (as he presumes) refracts more truly
then a concave reflects, [illeg] that he found no effect by one of six foot radius, w^ch [illeg]
till now he attributed to some other cause then the want of a good encheiria, as per
haps to the un |namely to the supposedly less| true reflexion of a sphæricall concave, & [illeg] \he supposed apprehended a/ [gr]eater difficulty of
describing a Parabola then an Hyperbola or Ellipsis. Nor could I well interpret the cause from [*] |* from w^ch many [good] experiments have been lost to have been other then the want of a good encheiria w^ch till afterwards appears not to have been wanting. I contend not that this was M^r Hooks meaning, but onely that his words seemed to import thus much, which gave me occasion to think there was no diligence wanting in making that experiment, especially since he expresseth that he made severall tryalls w^th it.|

And that you may not think I strained M^r Gregory's sense where he spake
of Hyperbolick & Elliptick Glasses & Speculums attempted in vain; I would ask to
what end those Speculums were attempted if not to compose optiqꝫ Instruments
w^ch is all I would infer from those words. For that those instruments if at all
attempted were attempted in vain is evident by the want of success.

This S^r I have said not that I desire to discourage the tryall of any
practicable way or to contend w^th M^r Gregory about so slender a subject \difference/. For I
doubt not but when he wrote his Optica promota he could have described more
fashions then one of these Telescopes & perhaps have run through all the possible
cases of them if he had then thought it worth his paines. Because M. Cassegrain
propounded his supposed invention pompously, as if the main buisini |e|ss was in the contri
vance of these \instruments/ I thought fit to signify that that was none of his contrivance, nor so ad-
vantageous as he imagined. And I have now sent you these further considerations