Measurement of Immersed Apertures. 



13 



Fio. 1. 



common stearine candle gives heat enough. The glass used should 

 be quite clean and always be held as near the point of the flame as 

 possible, in order to avoid the deposit of smoke. Large spherules 

 so made take an elliptical figure. Should they be required above 

 one-twentieth they are best formed thus : — Select a clean fragment 

 of window-glass, hrohen off (not cut), of such a bulk as will form 

 the desired sphere. Attach this by one corner, with heat, to the 

 point of a platinum or iron wire. Now rotate the mass while in a 

 state of semi-fusion by twirling the wire back and forwards between 

 the finger and thumb, holding it sometimes up or down, horizontally 

 or inchned, according to the way that the glass seems inclined to 

 sink. With very little dexterity spheres up to one-fifth of an inch 

 in diameter may be so obtained, the rotation of the wire enabling 

 the figure to be appreciated with some accuracy. When cool the 

 spheres are pulled off the wire, which enters but a little way. 

 These spheres are useless things enough alone. 



The next step is to convert them into piano lenses. First, make 

 a drill from a piece of round steel wire, about one-tenth diameter, of 

 the following form (Fig. 1). The end is filed 

 down as a square pyramid and hardened, using 

 the blow-pipe flame for the heat. The four sides 

 are then touched on the hone. This drill is best 

 used with the Archimedean stock, as it makes 

 a more truly circular hole. Now take a piece 

 of polished plate-glass, about one inch square, 

 and with the drill well moistened with turpen- 

 tine or coal oil, make several rows of conical 

 cavities in the face of the glass plate of gradually 

 increasing depth to the number of fifty or more. 

 The drill must not be used with too much 

 pressure, as the endeavour must be to get the 

 edges of the pits as sharp as possible. Having 

 cleaned the block, select from the stock of spherules such as appear 

 to match with the size of the holes in succession, lay them in proper 

 order on a card, then heat Fia. 2. 



your drilled block on a hot 

 plate and lightly smear a 

 piece of the best orange 

 shellac over it, so as nearly 

 to fill the cavities. Now 

 pick up your spherules one 

 at a time and drop each 

 into its appropriate hollow, 

 pressing it hard down with 

 a small wire tubular socket placed over the neck. When the block 

 is filled it resembles a small plantation of onions (Fig. 2). 



