101 
Auzout, himself a constructor of telescopes in France, on the ground that 
it was impossible to give that steadiness and firmness to its structure which 
were requisite for grinding glasses of any long focus; an effect which 
Hooke promised as a consequence of its adoption. In 1668 we find a 
description of machine by Mancini, which consists, however, only of a long 
arm, one end of which is fastened to the ceiling, while the glass is fixed 
to the other end worked ona plane. In 1676 Borelli was celebrated as a 
constructor of telescope-glasses, but we are not aware of the method which 
he employed. To 1719 belongs a work of Leutman’s, entitled, “Remarks 
on Glass-Polishing, in which are described improved machines for bringing 
glasses to greater perfection by the help of three motions.” This Work 
I have not seen, and cannot therefore be certain what is meant by the 
three motions here mentioned in the title. In 1741 Mr. Jenkins published 
in the Philosophical Transactions his method of grinding glasses spherical. 
It consists in making the glasses, fixed on the surface of a sphere, of which 
they are segments, revolve round one axis, while a hemispherical cap, which 
is fitted to the globe, and which grinds them, revolves on an axis at right 
angles to the former. This, though in appearance much different from 
Hooke’s machine, is nearly the same in principle, differing only in having 
the axis inclined at a different angle, and seems to be liable to the same 
objections. Huyghens’s machine is described in Smith’s Optics. It only 
serves to produce pressure, and to reduce the action of moving the glass 
backwards and forwards on the tool, to the action of turning a winch; the 
care of avoiding inequalities, by changing the relative position of the glass 
and tool, is left to the operator. Nollet’s machine, among the Machines 
Approuvées par Academie Royale des Sciences for 1733, (tom. VI. 
p- 127), is merely to give a rotatory motion to the tool by the feet, while 
the glass is ground upon it by the hand. 
Another machine for grinding glasses is described in the Machines 
Approwvées for 1736. It is by M. de Parcieux, and seems more likely than 
