SOUKCES OF ILLUMINATION 91 



pole as marked on the lamp ; otherwise the life of the lamp 

 is considerably shortened. 



Which of the wires making connection with the lamp is 

 the negative pole may be determined by using what is known 

 as pole-finding paper, to be obtained from any electrical 

 engineer's supply store. Or a simple method is to moisten a 

 piece of filter-paper with a solution of bromide or iodide of 

 potassium, and then quickly touch the paper with the ends of 

 both wires. Great care should be taken in doing this that the 

 ends of the wires are not allowed to touch each other, and they 

 should be so held that they are from an inch to an inch and a 

 half apart when they touch the paper. The negative wire 

 will then, where it touches, cause a small brown spot to appear, 

 and this is the end which should be connected up to the negative 

 pole as marked on the lamp. 



The type known as the Projector Nernst lamp should not 

 be selected ; in it there are three parallel filaments, but although 

 the arrangement is very suitable for ordinary optical projection, 

 it offers no advantage for microscopic work. 



Mercury Vapour. One of the more recent forms of electric 

 light likely to prove valuable in photo-micrographic work, 

 and in fact in all microscope illumination, belongs to the type 

 of which the mercury- vapour lamp is an example. 



In this method of light-production we have what is really 

 an arc light, but with the very important difference, that in the 

 mercury-vapour lamp the gaseous arc itself constitutes the 

 source of light, instead of one or both of the solid poles as 

 in the carbon arc. Further, the current necessary to main- 

 tain a mercury-vapour lamp is very much smaller than that 

 required to maintain any carbon arc. The mercury vapour 

 which constitutes the source of light is enclosed in a glass, or 

 preferably a quartz tube, and so is not subject to atmospheric 

 influence ; it may be exactly described as an hermetically 

 sealed mercury arc. 



The production of light in this case depends on the same 

 essential conditions as obtain in the arc light. The mercury, 

 however, constitutes the poles between which the arc itself 

 plays. The mercury is vapourised at the negative pole, and 

 the vapour becomes highly -incandescent as the result of its 

 molecular activity and not, as in the arc and other electric 



