88 PRACTICAL PHOTO-MICKOGKAPHY 



from the way in which the jet is usually supported, the light 

 is easily thrown out of adjustment. 



Whatever form of lime-light apparatus may be adopted, 

 it is advisable to see that it is of the very best make. The price 

 is usually somewhat high, but a reasonable outlay will be found 

 to repay itself, as otherwise unforeseen troubles are almost 

 sure to arise. 



Electrical Illuminants. These have many advantages 

 for the purpose in view. It is now so general for a supply of 

 current to be available in laboratories, and even in private 

 houses, that both on the score of convenience and of economy 

 one of the many forms of electric light is to be preferred. 



Among those that are suitable for photo -micrography are 

 the Nernst lamp, the mercury- vapour lamp, and the electric 

 arc. Ordinary incandescent filament lamps, either carbon or 

 their lately introduced substitutes, metallic filament lamps, 

 such as tungsten and tantalum, are totally unsuited for this 

 purpose. In these the filament is far too thin, and owing to 

 its length, the source of light is not sufficiently concentrated. 

 It also becomes very difficult to obtain critical illumination, in 

 which the image of the radiant is projected into the object - 

 plane of the microscope. Even though a diffusing medium, 

 such as ground-glass, be used between the light and the 

 microscope it does not answer well, for when the initial source 

 is of low intensity, as it is in all filament lamps, the loss of light 

 then becomes a really serious factor. 



Nernst Lamps. A convenient form is the Nernst electric 

 lamp, as its current consumption is small, and yet it has a com- 

 paratively high initial luminosity. It requires no special arrange- 

 ments for electrical supply, such as extra thick wires which are 

 necessary to carry the current of an arc lamp. Its fundamental 

 principle is that the metal oxides of certain rare earths will 

 not conduct electricity at normal temperatures, but will do so 

 quite easily if their temperature is first raised, the result being 

 a brilliant white light. Its spectrum is such that every 

 requirement is fulfilled when selective colour-screens are used. 



The filament, in the form of lamp which is to be recom- 

 mended, is a small straight rod varying in thickness according 

 to the current used. It will be advisable to consider only the 

 form which takes a current of one ampere, as in this the filament 



