VOL, I.] * PHILOSOPHICAL TRANSACTIONS. 23 



M. Auzout's Remarks on Mr. Hooks new Instrument for grinding 

 Optic Glasses; tvith Mr. Hooks Reply, N" 4, pp. 56 and 63. 



These two long papers contain a liberal controversy between these gentlemen, 

 concerning the powers and apertures of telescopes, with their methods of grind- 

 ing the glasses ; now of very little importance, and especially as the papers only 

 give account of two books, which are in the hands of tlie learned, viz. Hook's 

 Micrographia, and Auzout's Reply to part of it. 



Mr. Hook having, in that book, described a new engine for grinding optic 

 glasses of very great lengths, M. Auzout, in a small French tract, states several 

 difficulties and objections to it. He also thinks it impracticable to make any 

 glasses of above 300 or 400 feet at most, fearing that neither matter nor art 

 will be able to go even so far, and which consequently will be very far from 

 showing us either plants or animals in the moon, as some had pretended. He 

 farther proposes remedies for some of the inconveniences of the turning-engine. 

 To all which Mr. Hook here repHes, answering the objections, and rejecting 

 the proposed expedients. 



To illuminate an Object in any Proportion desired : and of the Dis- 

 tances requisite to bmm Bodies by the Sun. By M. AuzovT^ N" 4, 

 p. 68. 



One of the means used by Mr. Auzout to enlighten an object in any propor- 

 tion desired, is by some large object-glass, by him called a planetary one, be- 

 cause by that he shows the difference of light which all the planets receive 

 from the sun, by making use of several apertures proportionate to their distance 

 from the sun, provided that for every nine feet draught, or thereabouts, one inch 

 of aperture be given for the earth. Doing this, one sees (says he) that the light 

 which Mercury receives is far enough from being able to burn bodies, and yet 

 that the same light is strong enough in Saturn to see clear there, since it ap- 

 pears greater in Saturn than it doth upon our earth when it is overcast with 

 clouds; which, he adds, would scarcely be believed if by means of this glass it 

 did not sensibly appear so. Of which he promises to speak more fully in his 

 Treatise on the usefulness of great Optic Glasses; where also he intends to de- 

 liver several experiments, touching the quantity of light which is necessary to 

 burn bodies ; he having found, that not abating the light which is reflected by 

 the surfaces of the glass, about fifty times as much light would be necessary as 

 we have here for burning black bodies; and near nine times more for burning 

 white bodies than for burning black ones ; and so observing the intermediate 



