86 Incandescent Lamps for 



be the best, and would give a light lasting from twenty 

 minutes to over an hour, according to the amount of solution 

 that the cells were capable of holding. The next use to which 

 the small incandescent lamp has been recently and success- 

 fully applied is to the microscope. 



Mr. C. H. Steam, who has been associated with Mr. Swan 

 during the whole of his experiments with incandescent 

 lamps, has just introduced several forms of lamps and 

 apparatus for the purpose, and I cannot do better than to 

 quote from a paper read by Mr. Stearn before the Royal 

 Microscopical Society in January last. 



"The length of the incandescent filament is T T oth of an inch : 

 its diameter, Teeth of an inch; and its superficial area, about 

 ai^th. of a square inch. Two Bunsen cells, or four 

 Leclanche's, are sufficient to render them fully incandescent ; 

 but for general purposes it will be best to use an additional 

 cell, regulating the intensity of the light by means of the 

 adjustable resistance attached to the base of the microscope. 



" As the duration of the lamps is in an inverse ratio to the 

 temperature at which they are maintained, it is desirable 

 that the most intense light that the lamp will give 

 should only be employed for a very short time, when a 

 special effect is required — such, for instance, as for purposes 

 of micro-photography. If the lamp is at other times used 

 no brighter than is necessary to obtain a white light, and 

 the current turned off when observation is not going on, 

 the lamps will last a very long time, as experience has 

 shown that a life of more than 2000 hours of continuous 

 and brilliant incandescence is frequently exceeded by Swan 

 lamps. It is possible to obtain a light of 2J candles from 

 the tiny surface just mentioned, with an electro-motive 

 force of 3| volts, and a current of 1J amperes. It would, 

 however, at a safe temperature, give a light equal to one 

 candle. 



" As the source of light is almost a point, and the lamp can 

 be brought very nearly into contact with the slide, a higher 

 degree of obliquity of the illuminating rays can be 

 obtained than by almost any other method ; and hence 

 black-ground illumination is shown with great beauty, and 

 many of the diatoms display diffraction colours with 

 unusual splendour. 



"The resolution of test objects becomes very much simpli- 

 fied, as most of them can be resolved by the lamp alone, 

 without any accessory apparatus." 



