1829.] On the Manufacture of Optical Glass. 240 



formed by the adhesion of a particle of dirt or grit), it should 

 be examined, and removed by the point of a knife, if necessary, 

 and its place also marked. All these places and the whole 

 surface of the plate should then be examined for holes by a 

 still stronger test, namely, by holding the sheet of metal before 

 and close to a bright light, as a candle .or lamp, in a dark 

 room, and every hole observed, marked. In making this ex- 

 amination, it must be done carefully and minutely, holding the 

 plate in different directions to the light (for sometimes the holes 

 are oblique), and being careful that no reflexion from illumined 

 objects, as the hands, on that side towards the face shall 

 give deceptive indications. In the marking, too, the indicating 

 spot should always be made at a certain distance from the hole, 

 as the fourth or the third of an inch, and on the same plate 

 constantly in the same direction or towards the same edge ; the 

 holes are then easily found again, and the mark remains during 

 the soldering to guide the operator. 



32. The holes discovered by these examinations are to be 

 closed by little patches of platinum soldered with gold ; for gold, 

 like platinum, may be safely used in these experiments, when 

 reducing matter is absent. The gold has been used in the 

 finely divided state in which it is obtained by precipitation from 

 its solutions by means of sulphate of iron, but it must be washed 

 perfectly pure ; the patches are formed by cutting a piece of 

 clean new platinum foil into small square or rectangular plates : 

 a sufficient heat can usually be obtained by the use of the spirit- 

 lamp and mouth blowpipe. In the process of soldering, a little 

 of the powdered gold is heaped upon the hole and slightly 

 flattened by some clean instrument, the spirit-lamp is applied 

 underneath for a moment, which causes the gold to adhere 

 slightly, a selected patch of platinum is laid delicately upon the 

 gold, and then the heat of the spirit-lamp, urged by the blow- 

 pipe, is directed beneath against the place. Usually the gold 

 will melt and run instantly, the platinum patch will come into 

 close contact with the plate, and the operation will be completed. 

 If well done, the fused gold will appear all the way round in 

 the minute angle formed by the edge of the patch, and also 

 faintly at the hole on the opposite side of the plate. 



33. Sometimes, when the patch is large, or in the middle of 

 a plate, the heat obtained as above is hardly sufficient to melt 



