LECTURE AND CLASS-ROOM. 143 



The next source of light is the electric arc, as formed between 

 electrodes of carbon. This, under good conditions, approaches to 

 a fourth of the brightness of the sun, or about 47,000 times the 

 brightness of a standard candle. Here, then, we have a splendid 

 source of light, and with this great advantage, that the heating 

 rays which accompany it are comparatively few. It is mainly a 

 question of cost and complexity of apparatus. In the present state 

 of electric engineering we should require a gas- or steam- 

 engine to drive a dynamo, and then either take the light direct 

 from the machine, or make use of a secondary battery in which 

 the electric energy can be stored and used as required. If we 

 adopted the former plan, we should be almost sure to have a very 

 variable light ; and if the latter, should run the risk of an entire 

 collapse from the escape of the electric fluid. In either case, we 

 must provide machines and apparatus that are necessarily not only 

 very costly, but very cumbersome. The time may come when 

 electric light shall be supplied as gas is now supplied, and then we 

 may hope successfully to use it in our microscopic lanterns. But, 

 at present in Bath, the electric light is not available, though a 

 constant source of light might be secured by utilising the water 

 which runs to waste over the weir at the old city mills. 



As a source of Hght, we next come to the Drummond, or 

 lime-light, but its value is very small as compared with sunlight or 

 electric light. In its most effective form it gives a light equal to 

 about 450 candles, but in general it does not exceed 150 candles. 

 We need hardly mention any of the forms of oil or gas lamps, 

 as few, if any, of them would give us more than a 50-candle 

 power. 



It appears, then, that for the present we are confined to 

 the use of the Hme-light in its best form — that is, to the use of 

 lime, magnesia, or some other earth rendered incandescent by a 

 jet of oxygen and hydrogen gases, mixed before they issue from the 

 blow-pipe. Next, how can this light be most usefully applied so 

 as to produce on the screen a magnified image of the microscopic 

 preparation? It is impossible to transmit to the screen a greater 

 amount of light than falls on the area within which the object is 

 placed. Let us suppose this to be a circle of half-an-inch in diameter, 

 and the screen that we desire to fill nine feet in diameter. It is 



