92 PHOTOMICROGRAPHY 



than one filament, or with heaters coiled round a single 

 filament, but these cannot be used for critical work, as the 

 spirals of the heater give the appearance of dark bands 

 across the light when focussed on to the object plane. 

 There is no need to use a lamp fitted with an automatic 

 heater, as it is quite easy to start the light merely by 

 warming the filament with a spirit-lamp. Nernst lamps 

 are obtainable for both direct and alternating current, 

 and the candle-power may be taken as approximately 

 the same as the voltage oi the supply. They are 

 connected up in series with a resistance suited to the 

 voltage. Their one great drawback is that the filaments 

 are rather delicate, but they can be replaced 'at small 

 expense. 



Fig. 26 shows a very convenient mounting for a Nernst 

 lamp made by Zeiss. It is fitted to a saddle stand for 

 their triangular optical bench, and carries a short focus 

 aplanatic condenser and iris diaphragm. The screw (3) 

 is used to focus the condenser, and (4) to raise or lower 

 the filament which is shown in its mounting in the smaller 

 figure. No horizontal movement is required on account 

 of the length of the filament. When the lamp is placed 

 at about 30 inches from the microscope, the image of the 

 filament projected by the condenser is sufficiently large to 

 cover the whole of the back lens of the substage condenser. 

 Another convenient mount for a Nernst lamp is shown 

 at Fig. 8. 



Mercury Vapour Lamp. The spectra of all the fore- 

 going radiants consist of a continuous band of colours, 

 similar to the well-known solar spectrum, but varying in 

 the proportions of rays of different colours. Incandescent 

 vapours, on the other hand, give spectra consisting 

 of isolated bands or lines in various positions, depending 

 on the nature of the vapour. Lamps in which an in- 

 candescent vapour forms the source of light are very 

 useful when monochromatic light is required, as it is a 



