SOUKCES OF ILLUMINATION 77 



however, has used an oil lamp of simple construction for many 

 years which has been found in every way satisfactory. The 

 oil reservoir is a tin chamber about six inches square and one and 

 a half inches deep. The burner is a circular one of the very 

 best make procurable, and the light which it gives is sufficient 

 for any work except when the use of oil-immersion objectives is 

 necessary. No provision is made for vertical adjustment, as 

 with such a large source of light it can be placed in position in 

 relation to the microscope once and for all. With all oil lamps 

 it should be remembered that the heat emitted is considerable ; 

 so the lamp should be placed as far from the microscope as 

 possible, and it is always advisable to interpose between the 

 microscope and lamp, but as near the latter as possible, a 

 blackened metal diaphragm of fairly large size with a hole 

 in it about three-quarters of an inch in diameter. 



The oil used should be of the very best quality, and should 

 have from 2| to 3 per cent., by weight, of camphor added to it, and 

 dissolved in it. The wick itself should be evenly trimmed and 

 thoroughly dried before it is inserted in the wick holder. All 

 parts of the lamp should be kept thoroughly clean, as on this 

 depends entirely the efficiency of the light and its freedom from 

 smell. The glass chimney should preferably be of Jena glass, 

 to lessen the risk of fracture from changes of temperature. 



The chief objection to an oil lamp is, of course, that it 

 necessitates prolonged exposures, especially where the work 

 being done is of magnification exceeding 500 diameters. It 

 must not, however, be overlooked that some of the best photo- 

 micrographers have obtained excellent results by its use, and 

 some have even gone so far as to recommend it for all classes 

 of work. 



The duplex form of burner is to be avoided, as, although a 

 higher degree of incandescence is obtained owing to the close 

 proximity of one flame to the other, yet the inevitable dark 

 space that occurs between the two makes its use very difficult 

 when critical work has to be done with the edge of the flame, or 

 where an image of the radiant in the object-plane is desired. 



Welsbach Gas. The Welsbach gas light, although it has a 

 higher illuminating power than an oil lamp, suffers from a very 

 considerable disadvantage ; for in all attempts at using this 

 source of light for critical work, the image of the incandescent 



