cnsiAcr'A, bj

Dr. Hook. if>.t- 6?.

To \n' fsurc Auft.ifrcs tt oneSutton ; by M.Auzout. n. 7.,t-

^ST

( ' 9 * )

z. This Theory oF Apertures, scems ro nie not very clear. For the fernedass will endure greater or lesser Apertures, according to the lesser or greaterLight of the Object ■' 11 it be for the looking on the Sun orVcmts, or for feeingthe Diameters of the fixed Stars, then (maller Apertures do better; if for theIVloon in ihe Day light, or on-Saturn, or Jupiter, or Mars, then the Lar-gest. ’l’hus I have osten made ufe of a n Foot Glast to look on Saturnwith an Aperture of almost z inches, and with a lingle.Eye Glaß of 2 . inchesdouble Convex ; but, when with the ferne Glafs I looked on the Sun or Venus,1 ufed both a Imaller Aperture, and lhallower Charge.

XIV. I'havtfound longfincea way to measure, with a great Teleseope, thedistance of Objects upon the Earth front one Station. The Practiceindeed doesnot altogether an (wer the Theory, bccaule that the Length of the Telefeopesadmits of (ome Latitude ; yet one comes near enough, and perhaps as just asby rnost ot the ways ordinari !y ufed with Instruments. That, which 1 amproposing, I doubt not but Mr. Hook, will löon understand, and fee the Deter-mination ot all Cafes poilible. 1 shn.ll only lay, that it we look upon thelble Theory, we may ntake ufe ot an ordinary Teleseope, whereof the EyeGlaß is to be Convex : For by.putting the Glasses at a little greater distance,than they are, porportionablyto the distance fot which it is to ferve, and byadding to it a new Eye Glaß, the Object will be scen distinct, though Ob-leute ; and if the Eye Glaß be Convex, the Object will appear Erect. Theymay be donc two mannet of ways ; eithcrbyleaving the Teleseope in its or-dinary Situation, the Object Glaß before the Eye Glals ; or by lnverting it,and putting this before that. But if any will malte ufe of two Object Glasses,whereof the Focufes areknown, the Distance of them will be ltnown. If itbe fuppofed, that.theTocus of the first beB, and that of the fecond C, andthe distance given, Bst iD, and that D — C, be equal to F; for thisdistance will be.equal to B ■+ C + F — r F 2 C 2 . And if you havethe Focus of the first Object Glaß, equal to B, the distance where youwill put the fecond Glaß equal . to B + C D, the Focus of the

CD

fecond Glaß will be found equal to-. And if you will that the Ob-

1 CH-D 1

ject {hall be magnified as much with thefe two Glasses, as it would be with aiingle one, whereof the Focus should be of the distance given, having the Fo-cus of the Object Glaß given equal to B, and the distance given to B H- D;

the distance between the first and fecond Glafs will be equal to

i B 2 -+-i bdiB -h D~’

whence fubducting B (the Focus of the Object Glafs given) therc remainsBD

B~-+D ' an ^ ^ t ' 1 ' 6 ^ um fuppofed equal to C, we {hall easily know, by

the precedent Eule, the Focus of the fecond Glaß.

XV. Frep.tr»