Mr Hookes answer to Mr Beal
Sir
The Hevelian Polemoscope, upon Examination, I find much the same with what Hevelius has described in his Selenography, but a little out of order, and the glasses not good: nor indeed was the instrument, that I can find, ever brought to any great perfection by that worthy Author, tho his invention be very good. For, I remember, about 8 or 9 years since, when I first saw and examined one of those instruments, which was said to be one of the Authors, I found it very deficient in one particular, and that was that one could see but a very little angle at once, which is a most insufferable defect in that kind of instrument, whereupon I remember, I did then bestow some thoughts on it, and reduced it to that passe, that tho the Tube were 20 or 30 foot long, and the whole at the end not above 2 foot over, yet I was able to see as great an angle by looking in at the lower hole of it, as if my eye had been placed at the very top of the Tube within an inch or two of the uppermost hole, and in making tryals of that kind, I remember, I found out many other pretty uses and contrivances, which I ghesse Hevelius hardly thought of. But as for those Hevelius applyes it to, I fear it will never be of any great use, especially in Naval affairs. Your microscope I found a pretty good one, and better than usual. The aperture of it was a litle too small, and that made it appear a litle dark, but I opened the aperture somewhat, and that made it appear much brighter, and will perhaps please you much better, than the other aperture did.
That a Tube of 2 1/2 foot should show the hornes of Venus when in her perigean, and neer conjunction with the sun, is no wonder; for, she may then be very definitely seen with a glass of less than 2 foot, as I have severall times observed. But those Tubes, which I have hitherto seen of Mr Smetwicks, have been very good of the kind, and able to bear as great an aperture and charge, as any kind of perspectives of that length, I have yet met with. But both for Telescopes and Microscopes, we are yet very deficient, and come far short of that perfection, which our Theory's would encourage us to. But I am not quite out of hope, but that, had I leasure and opportunity, some advance might be given to both kinds, by yours
Rob. Hooke
Entered LB.Suppl.1.pag.451.