252 On the Manufacture of Optical Glass. [1829. 



perpendicular to the bottom of the tray, but a little outwards 

 above, and the proper degree of inclination is easily given by 

 using a mould upon which to bend the corners. This should 

 be a thick square piece of wood having the four corners cut 

 with different degrees of obliquity : when the corners of the 

 tray are first imperfectly formed, it will be easy to ascertain by 

 trial, which corner of this mould 

 will give the obliquity and position 

 already described as necessary, , 

 after which the folding may be 

 easily finished upon it. The ac- 

 companying sketch represents first 

 a good and then a bad folding. 



38. All occasion for changes in the folds, especially at the 

 corners, should be avoided. The folds should be decided 

 upon as the work proceeds, so advantageously as to make 

 alterations unnecessary. The closer the corners are pressed, 

 the smaller is the quantity of glass contained in them, and the 

 less risk is there of the platinum being broken when the finished 

 glass is taken out ; but it is proper to avoid general contact 

 betw r een the corners and the sides against which they are dis- 

 posed, otherwise welding is likely to occur during the stirring, 

 and the platinum is injured for future experiments. 



39. The tray being formed is again to be examined for holes, 

 first by a light as before (31), and then in the following 

 manner : Being laid upon a sheet of bibulous paper, alcohol 

 is to be carefully poured in until the fluid is within the fourth 

 or the sixth of an inch of the lowest edge of the tray, so as to 

 occasion no running over at the sides or corners. If a large 

 hole exist, it will be rendered visible immediately ; but if none 

 such appear, a large basin or some other cover is to be placed 

 over the tray to prevent evaporation, but without touching the 

 vessel or its contents ; and the whole is to remain undisturbed 

 for some hours. Being then examined, the wetting of the 

 paper will indicate a hole or a badly-folded corner, and will 

 point out the faulty place : the tray may easily be shifted from 

 one part of the paper to another for the discovery of any 

 moistened places beneath. Sometimes holes occur so small 

 that alcohol will not run in a sensible quantity through them. 

 Suspected places of this kind and suspicious corners also should 



