11 8j USE^vi; NOTICJf*. 



M°erior T to 9 " ^ m ° r<? un ^ ual ln ^ 9VY#ty in different parts of the same 

 cast. PM te > tlian the blown glass. It woujd not be difficult to ex- 



plain this from the circumstances of the making; but the 

 principal object of the present notice is, to mention that he 

 succeeded in making a thick lens out of plate one quarter of 

 Blown p^ate an inch in thickness, by softening the glass by heat upon an 

 g 'o S k d™ 7 ^ eartnen momd of tr, e proper curvature, and upsetting or pres- 

 thicker. &'wg the borders inwards, (taking care to avoid folds or wrink- 



les,) till the. edge was an inch thick, and the diameter five 

 inches. He then surrounded the gJass by a metallic ring of 

 six inches diameter, and three quarters of an inch deep. Within 

 this ring he again heated the glass, upon which he previ- 

 ously placed an upper convex earthen mould. The glass 

 A lens, made thus obtained appeared very goocl, and when ground and po- 



sn this manner, } ished enao led him to make a triple object glass of seven 

 was excellent ; r J ° 



feet locus, producing, as he says, a much greater effect than the 



glasses of Dollond, but without admitting of a proportionate 

 aperture. For the lenses of that celebrated artist bore an aper- 

 ture of 42 lines, and his lenses would not admit of more than 

 4 inches or 48 lines; which, however, adds more than one 

 third to the whole quantity of light. From the great care 

 ki working, he did not think that the external parts of the 

 fcutthebor- lens were defective on account of the figure. Tiie defect 

 probably from ar0:je most P r °bably from the flexure and contortion of the 

 the glass aud grain of the glass in pressing in. For an ingenious philoso- 

 not t e figure, ^ical artist has assured me, that there is great difference in 

 lenses and prisms made of the clearest plate glass ; accordingly 

 as the line of vision is directed at right angles to the natural 

 plane, or more obliquely or coincident with it, the latter being 

 in general good for nothing. Whence, and . from other 

 facts, he inferred that the layers of glass plates differ consi- 

 derably in their densities. 



