426 BUNSEN'S PHOTOMETER, 



The grease-spot is made with stearine : tallow or oil produces a 

 yellowish and fatty spot, which soon becomes dirty by adhering 

 particles of dust. The spot must be very faint if it is to be rendered 

 invisible when the illumination is equal. Let one or two drops of 

 stearine from a burning candle fall upon paper, and after a little 

 time remove cautiously with a knife the solid stearine which has not 

 penetrated the paper. The spot is now too strong, but it may be 

 rendered fainter by placing the paper between a few layers of blot- 

 ting-paper, and putting a hot smoothing iron on it for a short time 

 on the top. The stearine is thus again liquefied, and a portion of 

 it is absorbed by the blotting-paper. By repeated trials it will soon 

 be found how long the hot iron should remain upon the paper, so 

 as to produce a spot of the proper kind not too strong nor too 

 faint ; for in the former case it will not disappear when equally 

 illuminated, and in the latter case it will not be distinct when 

 the illumination is unequal. The paper when thus prepared is 

 glued upon a small frame of wood, with a short handle, made by a 

 joiner, or if needful a cardboard frame, fixed upon a wooden handle 

 prepared with a knife, may be used. The handle should be of a 

 thickness which allows of its being conveniently fixed in a candle- 

 stick, and that side of the frame upon which the paper is stretched 

 should be exactly over the middle of the handle, as shown at a a, fig. 

 242, B. A straight line is drawn with chalk on a long table ; the 

 lights to be compared are placed upon the ends of it, and the screen 

 moved to and fro upon the line. The distances are in this case 

 also most conveniently measured by drawing circles with chalk 

 round the three supports of lights and screen, removing the latter 

 after the experiment, and measuring the distances of the centres 

 of the circles from one another. 



For the experiment with the four candles (fig. 242, J.), a suitable 

 support for the candles may be prepared from a piece of sheet-tin, 

 i cm wide and 13 cm long. Cut off at each corner a square piece, 5 mm 

 each way, punch five holes, as shown in fig. 242, 6\ and bend 

 each side 5 mm from the edge, so as to produce a shallow tray. Four 

 tacks, 1'5 or 2 cm long, with flat heads, are thrust from under- 

 neath through the holes a, &, d, and e, and soldered to the tray ; this 

 is done by moistening each tack with a drop of soldering fluid, 

 placing close to each a small piece of solder, and heating the whole 

 over a gas or spirit flame, until the solder flows into the space be- 

 tween the head of the tack and the tray. Another tack is driven 

 through the hole c in a contrary direction ; it serves for fastening 

 the tray to a wooden handle, while the four spikes formed by the 

 other tacks serve for fixing the candles upon them. The turned-up 

 edge of the tray prevents the stearine from running down at the side. 



