152 PRACTICAL TEEATISE ON THE MICROSCOPE. 



wick, instead of a coarse screw- The pinion is attached near 

 the lower end of the burner, and is turned by a milled 

 head, but from the circumstance of the pinion working in the 

 oU, it is found that, after a httle use, the oil will escape be- 

 tween the pinion and the collar in which it works, and will 

 be continually dripping. These lamps are provided also 

 with very long chimneys, and the gallery which supports the 

 chimney is made to slide up and down the burner, so as to 

 diminish or increase the intensity of the light ; but by having 

 a lamp of the form represented in fig. 107, the long chim- 

 ney and the risk of leakage are done away with. The only 

 inconvenience in the use of the fountain lamp is that the 

 reservoir may, by mistake or accident, be pidled up when 

 nearly full of oil, and it sometimes happens, on returning it to 

 its place, that a considerable quantity of oil will escape ; 

 this wiU, therefore, raise the level, and the wick receiving 

 more than it can consume, the excess will escape by the 

 interior and exterior tube of the burner, and the cup at the 

 bottom to receive this excess will speedily fill and overflow. 

 The first indication of this occurrence wiU be given by the 

 elongation of the flame, and by its smoking, in consequence of 

 the holes in the cup, which admit the air into the interior of 

 the flame, being stopped up. 



To ensure a good light from a lamp, many things must of 

 necessity be attended to. The lamp should be perfectly 

 clean, and the wick so long as to be at least one inch above 

 the brass to which it is attached. The tube through which 

 the air passes to supply the interior of the flame should be free 

 from portions of charred wick, which often lodge in it, and the 

 holes in the cup at the bottom of the burner must not be 

 covered with oU. The oil itself ought to be the best sperm, 

 and no lamp of the Cambridge kind should be put aside 

 for a long time, but should be occasionally burnt. Those 

 on the fountain principle will burn well even after having 

 been out of use for some months, as the oil is kept con- 

 tinually on a level with the top of the wick, but in the 

 other form, when the reservoir is not full, the oil has to find 

 its way to the top of the wick by capillary attraction. 



