DOI Seite / Zitierlink:
https://doi.org/10.11588/diglit.68888#0030
The Preface.
means,the longer the Glaft be, the bigger aperture will it bear,is the Glaffes
be os an equal goodneft in their kind. Therefore a fix will indure a
much larger Aperture then a three seet Glaft; and a fixty foot Glaft will
proportionably bear a greater Aperture then a thirty,and will at much ex-
cel it also as a fix foot does a three foot, as I have experimentally obfervd
in one os that length madeby Mr. Richard Reives here at London,
which will bear an Aperture- above three inches over, and yet make
the Object proportion ably big and disiinEl; whereas there are very
few thirty foot Glafsesthat will indurcan Aperture of more then two in-
ches over. So that for Telescopes, fuppofmg we had a very ready way
os making their ObjeEi Glasfesof exaEtly fyherical Surfaces, we might, by
increafing the length) of the Glaft, magnisie the ObjeEi to any afsignable big-
neft. And for performing both thefe, I cannot imagine any way more ea-
sie,andmore exaEi, then by thus following Engine,by means of which, any
Glafses,os what length foever,may be sfeedily made.lt feems the mofi eafie, .
because with one and the fame Tool may be with care ground an ObjeEi
Glaft, of any length or breadthrequifite, and that with very little or no
trouble in sitting the Engine, and without much skill in the Grinder.
It seems io be the mofi exaEt, sor to the very lafi firoke the Glast does
regulate andreEiisie the Tool toitsexaEi Figure; and the longer or more
the Tooland Glaft are wrought together, the moreexaEl will both of them
be of the desir d Figure. Further, the motions of the Glaft and Tool do
so croft each other, that there vs not one point os cithers Surface,but has
ihoufands of croft motions thwartingit, so that there can be no kind os
Fings or Gutters made either in the Tool or Glaft.
The contrivance of the Engine is, only to make the ends of two large
Mandrils so to move, that the Centers of them may be at any convenient
distance asonder, and that the .Axis of the Mandrils lying both in the same
plain produc’d, may meet each other in any assignable Angle y both which
requisites may be very well perform’d by the Engine describ’d in the third
Figure of the first Scheme : where A B lignifies the Beam of a Lath fixt per-
pendicularly or Horizontally, C D the two Poppet heads, fixt at about two
foot distance, E F an Iron MandriLyfaofe tapering neck F runs in an adapt-
ed tapering brals Collar; the other end E runs on the point of a Screw G 5
in a convenient place of this is fastned H a pully Wheel, and into the end of
it,that comes through the Poppet head C, is screweda Ring of a hollow
Cylinder^, or some other conveniently ssiap’d Tool, of what widenels (hall
means,the longer the Glaft be, the bigger aperture will it bear,is the Glaffes
be os an equal goodneft in their kind. Therefore a fix will indure a
much larger Aperture then a three seet Glaft; and a fixty foot Glaft will
proportionably bear a greater Aperture then a thirty,and will at much ex-
cel it also as a fix foot does a three foot, as I have experimentally obfervd
in one os that length madeby Mr. Richard Reives here at London,
which will bear an Aperture- above three inches over, and yet make
the Object proportion ably big and disiinEl; whereas there are very
few thirty foot Glafsesthat will indurcan Aperture of more then two in-
ches over. So that for Telescopes, fuppofmg we had a very ready way
os making their ObjeEi Glasfesof exaEtly fyherical Surfaces, we might, by
increafing the length) of the Glaft, magnisie the ObjeEi to any afsignable big-
neft. And for performing both thefe, I cannot imagine any way more ea-
sie,andmore exaEi, then by thus following Engine,by means of which, any
Glafses,os what length foever,may be sfeedily made.lt feems the mofi eafie, .
because with one and the fame Tool may be with care ground an ObjeEi
Glaft, of any length or breadthrequifite, and that with very little or no
trouble in sitting the Engine, and without much skill in the Grinder.
It seems io be the mofi exaEt, sor to the very lafi firoke the Glast does
regulate andreEiisie the Tool toitsexaEi Figure; and the longer or more
the Tooland Glaft are wrought together, the moreexaEl will both of them
be of the desir d Figure. Further, the motions of the Glaft and Tool do
so croft each other, that there vs not one point os cithers Surface,but has
ihoufands of croft motions thwartingit, so that there can be no kind os
Fings or Gutters made either in the Tool or Glaft.
The contrivance of the Engine is, only to make the ends of two large
Mandrils so to move, that the Centers of them may be at any convenient
distance asonder, and that the .Axis of the Mandrils lying both in the same
plain produc’d, may meet each other in any assignable Angle y both which
requisites may be very well perform’d by the Engine describ’d in the third
Figure of the first Scheme : where A B lignifies the Beam of a Lath fixt per-
pendicularly or Horizontally, C D the two Poppet heads, fixt at about two
foot distance, E F an Iron MandriLyfaofe tapering neck F runs in an adapt-
ed tapering brals Collar; the other end E runs on the point of a Screw G 5
in a convenient place of this is fastned H a pully Wheel, and into the end of
it,that comes through the Poppet head C, is screweda Ring of a hollow
Cylinder^, or some other conveniently ssiap’d Tool, of what widenels (hall