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with a hole' in the midst of the Object Metal, to transmit the Light to anEye Glast placed behind it : I had thence an occafion of conlidering that Ibit-'of Construction«, and found thele Dilädvantages in it; vif. i. There will bemore Light lost in the Metal by Reflexion froni the little Convex Speculum,-than fromthe Oval Plane. For it is an obvious Oblervation, -That Light ismost copiousty Reflected from any Substance when Incident most obliqucly.-i. The-Convex -Speculum will not Reflect the Rays lö truly as the Oval-Plane, unlesi it be of an Hypcrbolick Figure; which is incomparably morerdifficult to form than a Plane; and if truly formed, yct would only Reflectthole Rays truly, which respect the Axis. z. The Errors of the fäid Con-vex will bi much augmented by the too great distance, through which the-Rays Reflected from it must pals, before their arrival at the Eye Gials. Forwhich realem, I find it convenient to malte the Tube no wider than is nccest.fary, that the Eye Glast be placed as ncar to the Oval Plane, as/is pollible,-wichout obstructing any uleful Light in its paifage to the Object Metal.4..; The. Errors. of the Object Metal will bc. more . augmented by Reflexion-from the Convex than from the Plane, becaulc of the Inclination or Deflcxion»of the Convex on all fides, from the Points on which every Ray ought to beIncident. 5. For thde Realöns there is requisita an extraordinary Exactitefs -in the Figure of the little Convex, whcreas I find by Experieuce, that it k.much more djflicult to communicate an exact Figure to stich. Imull pieces of.Metal, themto thole that are greatcr. 6. Bccaule the Errours at the Perl.meter of the Concava Object Metal, caufed by the Sphericalnels of its Figure,,are much augmented by the,Convex, it will not with distinctnefs bear <6-large an Aperture as in the other Construction. 7. By realbn that the little.-Convex conduces- very muck to the .Magnifying Virtue of the Instrument,which the Oval Plane dath not, it will Magnisie much more..in proportio»,to the Sphere, on which the great Concave is ground, than in the other,delign ; and 1 b Magnifying Objects much more than it ought to do in pro-,portian.to its Aperture, it must reprclent them very obscure and dark; and.not only lo, ,but. also confuled, by realbn- of its being ovcr-charged. Nor is-there any convenient Remedy for this. For ds the little Convex be made ofa Jarger Sphere, that will caule a greatcr inconvenience, by intercepting teo-naany of the best. Rays ; or if the Charge of the Eye Glast be made so muck-Shaliower as is neeeffary, the Angly of Vision will thereby bccome lb little.,that k will be very difficult and troublelbme to sind. an Object, and of that/Object when found there will be hüt a very Inrall part seen at once..

By this you may-pereeive, that the. three Advantages, which . Mr. Cnsfc-vyain propounds to. himlelf, are rathet Dilad.vantages. For according .to hiv rDesign, the Aperture of the Instrument will be but Imall, the Object dark;and confuled,. and also difficult to be found» Nor do I leo, why the Re-flexion k more upon the läme Axis, and (a more Natural in one cafe-than in.»the othec : since the Axis it (elf is Reflected towards tha Eye by the. Oval,Plane; and the Eye may. be defended from external Light), as well at the Side_.as at the Bottom of the Tube_.

Mr. Gregor-/