'V ilfirturnani Charles ofthcfc Inliru-

mnls\ly

. Newton.ii jj. }'. *03-

( 100 )

Here is made another Instrument like tlic former, which does very weil.Yellerday I comparcd it with a fix Foot 1 elelcopc, and tound it not only taMagnitie more, but allbmore Distinctly. And to day 1 {ound, that I couldReust in one of the Philolöphical Tranlactions, placed in the Sun's Light, atan hundred Foot Distance, and that at a hundred and twenty Foot Distance1 could di'cern some es the Words. Whcn I made this tryal, its Aperture(dehnest next the Eyej was equivalent to more than an Inch and a thirdpart of the Object Metal. This niay ’bc of some ule to those that {hall«ndeavour any thing in Reftexions; for hereby they will in iome meafurebe enabled to judge of the goodndsof their Instruments.

5-. I know that the Aperture was 1 f of an lnch, by trying that an Obsta-cte of that Breadth was requifne tointerccpt all the Light, which came frontone Point of the Object.

I ihould teil you altb that thelittle plainpiece of Metal, next the Eye Glast,

‘ is not only truly Figured : \Yhereby it happens, that Objects are not 16 di-stinct at the Middlc as at the Edges. And I hope, that by correcting itsFigure, (in which I find more difticulty than one would cxpect,i they wiilap-pear all over Distinet, and Distincter in the Middlc than at the Edges.And I doubt not but that the performances will then be greater.

But yet 1 find, that there is more Light lost by Reiiexion of the Metalwhich 1 have hitherto ust'd, than by Tranfrnillion through Glastes: Forwhich realbn a Shallower Charge would probably do bettcr for Oblcure Ob-jects ; liippofc stich a one, as would nrake itMagnisie 34 or 31 times. But forBright Objects at any distance,-it leems capable of Magnifying 38 or 40 times,with sufficient Distinctnels. And for all Objects, the läme Charge, 1 believe,anay with Advantage be allowed, if the Steely Matter, imployed at London,be more strongly Restective than this which 1 have uled.

The performances of one of thesc Instruments of any Length being known,it will appear by this following 'Fahle, what may be expected front thole ofother Lengths by this way, if Art can accontplifh what is promilcd by theThcory. In the first Column is expressed the Length of the Telefcope inFeet ; which doubled givesthe Scmidianteter of the Sphere, on which the Con-cave Metal is to be ground. In the lecond Column are the proportions ofthe Apertures for thole leveral Lengths. And in the third Column are theProportions of the Charges, or Diameter ofthe Spheres, on which the ConvexSuperficies of the Eye Glastes are ro be ground.

Lengths.

Aperture j .

Charges.

Lengths.

Apertures.

Charges.

2

IOO

IOO

8

800

ZOO

1

168

”9

10

946

2.1 I

z

7.83

I 4 1

ii

1084

ZZ1

3

Z8Z

16

rZ 45

238

4

476

168

2.0

iygi

zyi

5

562.

178

182.4

z 6 3

6

645

186