1829.] On the Manufacture of Optical Glass. 251 



necessary at the corners of the tray are especially likely to 

 render the same parts unable to bear a second and third 

 bending ; but the necessity of having them in the same place 

 may be usefully obviated by placing the gauge oblique to the 

 sides in one direction and in another, on different occasions, 

 and moreover gives other advantages in finishing the folding 

 of the corners (36). These attentions, tending to the preser- 

 vation of the platinum for repeated service, are very needful, in 

 consequence of the great expense of the material : the value of 

 the plate in question is about 61. 10s., and when worn out, it 

 may be sold for about half that sum. Whether it be used 

 therefore once, twice, thrice, or four times, makes considerable 

 difference in the expense of the resulting plates of glass. 



36. When the gauge is properly placed on the platinum, the 

 sides are raised perpendicularly : this produces four projecting 

 folded triangular corners, which being pressed close, are then 

 turned against the sides, and a square tray is finished, which 

 has no aperture or orifice below its upper edge. The folding 

 of these corners is a matter of much more consequence than 

 might be anticipated. The plate is seldom so regular that the 

 parts of two neighbouring sides which come together at a corner 

 are exactly of equal height ; neither is it desirable that it should 

 be so, and the unsymmetrical position of the gauge to the plate, 

 already recommended (35), is almost sure to prevent it. In 

 that case, of the two sides of the folded corner, one will be 

 higher than the other, and if the corner be so folded that its 

 lower side is towards the tray and beneath its edge, a kind of 

 siphon is formed which becomes charged with fluid by capillary 

 action, and continues to discharge glass from the tray during 

 the whole time of heating, notwithstanding that all the edges 

 are much above the level of the fluid within. This in a long 

 experiment is competent to occasion serious injury. 



37. I have found, even when the edges of a corner have 

 been of equal height, but below the edge of the side against 

 which they are disposed, that still this capillary and siphon 

 action has gone on, and the reason is not difficult to com- 

 prehend ; the corners therefore have always been folded in 

 such a manner, that their highest edge has been inwards, and 

 both their edges above the level of the corresponding edge of 

 the tray. To effect this, the line of their lateral flexure is not 



