| Title: | Copy Letter In Scribal Hand H?, John Beale To Hartlib? |
|---|---|
| Dating: | 28 February 1660 |
| Ref: | 67/22/5A-6B |
[67/22/5A]
Hereford 28 Febr. 59.
In your last of Febr.11 You aske my opinion, whether it bee possible to devise such Glasses by which a man may read at night without a candle. To mee it sounds like an æqvivocation, That Keus art may make weake Lamps sufficiently luminous. For is it possible to augment the light of any shining matter, as of dead woode, fish &c to such incredible use! For a better answere this secrete, I commit to your bosome, I can & have devised a perspective of the shortest kind, proper to helpe the sight in a very thic miste. The secret is this : The glasses (at least the object glasse) must bee of a brighter glasse, than is fit for Perspectives or for spectacles, not wildly glary nor scattering refractions, but eqvably & overstrongly, receiving the rayes of luminous bodyes. This perspective in cleare dayes gives the objects too staringly & flickeringly luminous; & in a very darke miste gives the bodyes of trees & men much distinctly than a truer or better Perspective; & then againe may bee fitted to eqvall or exceede those better Perspectives for all vses, by the only addition of a hoode, as I intend to shewe you by example. For if you please to send mee halfe a dosen or fewer concave eye glasses, some single concaves, some double concaves, some of the single concaves as narrowe & deepe as you can get them, & some of the double concaves of the largest segment & 2 or 3 of the broadest burning glasses; I will fitt them for the horseman, & seaman, & other uses, & soe returne you all your owne againe. In this [catchword: you]
[67/22/5B]
you neede not bee at charges for any costly workeman, but get the best that can bee got from some good workeman at ordinary rates, 6d a single concave, & 1s at the most for the burning glasse. I can give you the rules off proportion as I have done heretofore, but leave that to chance. Only send mee a payre or one whole glasse of any broken spectacles, that doe best agree with you eye, or the eye of any other persone for whom you intend them, & I doubt not of fitting severall Perspectives proper for all occasions; but you must expect no better materialls, than horne or woode. For wee have noe Ivory, nor workeman fit to handle it. If in the meane time you make use of Mr Figulus for some of De Keus Concaves & convexes, those you may fit in Ivory, or better mettalls, for more ornament. I hope Mr Woode will transcribe to Mr Figulus the Hevelian Polemoscope, which is a present fit for Gen. Monke or any Prince, & neede not bee of any valuable charge, if hee please soe to order it: & from thence Hee may find it most easy to order a Cane or rideing staffe (which are newely come againe in fashion) soe as the rider may see from the knob see all that is done behind his backe, & soe clearely, as through a Perspective glasse, much more clearely, than if hee looked backe with his owne eyes. This I cannot here direct, because wee have noe man that can make the specula plana è segmento elliptico to answere the concave & convexe. You may better [catchword: buy]
[67/22/6A]
buy the glasses, such as are ready made, than bespeake them, & rather at severall places, & then they may bee of severall kinds, & one or 2 spectacles the oldest that can bee found, as for persons of 100 yeares old, soe they bee of good glasse, & well turned. The circumference of the burning glasse for the horse man should bee about this proportion [A circle of radius approx. 7/8 inch is drawn here]. For the Seaman as much larger as you please. I have sometimes gotten good at 1s a piece with an Ivory bond, of which wee have noe neede./.
P.S. Sir If it bee reqvisite to direct by the rule of proportion, for our use the least proportion of the double concave should bee wrought upon a globe whose Diameter[altered] should bee neere five inches. & for fayrest tubes I could wish one double concave that fitted a sphære of five inches & a halfe in diameter. For a tube of one foote length the convexe should bee wrought on both sides, on a globe of 4 foote diameter. for a tube of 3 foote & halfe the diameter of the sphære to the double convexe should bee 8 foote. I shall not thinke of longer tubes for our use, nor without the helpe of a convex from from Keus from a sphære of 12 foote at leaste, & perfectly plaine [catchword: on]
[67/22/6B]
on the one side. Practically, if the sun shines, you may find the best convexes by the smallnes of the suns point of the true distance; & that well allso shewe what length of tube the convexe reqvireth./.