98 PHOTOMICROGRAPHY 



burner ; the chimney carries an iris diaphragm whose 

 aperture may be used as the source of light and focussed 

 instead of the mantle, in order to eliminate an image of 

 the structure of the mantle in the field of view. 



Incandescent Spirit, or Petrol Lamps. When gas is not 

 available a mantle may be heated by a flame from the 

 vapour of methylated spirit or petrol, which is forced 

 through a small jet, and mixing with air in the burner 

 forms a very efficient and convenient substitute for gas. 

 In some of the spirit lamps a small flame is used to 

 keep the spirit container warm enough to give off vapour 

 under sufficient pressure ; in others the pressure is main- 

 tained by occasionally blowing air into the reservoir from 

 a rubber bulb, as in the Meta lamp. The petrol lamps 

 require neither of these alternatives, as the vapour is 

 given off continuously when once the jet is hot. The 

 lamps are started by heating the jet, or small pipe 

 through which the spirit or petrol passes, by a spirit 

 flame, or by lighting a small amount of spirit placed 

 in a cup below the burner. 



The "Perfectol" and "Invetrol" are good petrol 

 vapour lamps with upright and inverted mantles 

 respectively. The simplest models can be hung from 

 a nail on the wall, or supported on a stand, as shown 

 in Fig. 46. Whenever petrol, methylated spirit, or 

 even paraffin are used for burning purposes there is 

 danger of fire, but with proper usage the vapour ]amps 

 are perfectly safe and they give a brighter light than 

 incandescent gas. The small petrol lamp shown at 

 Fig. 46 gives about 50 c.p., while larger ones give from 

 100 to 300 c.p. The chief difficulty is the heating up 

 previous to lighting. This takes two or three minutes, 

 and care must be taken that the jet is thoroughly hot. 

 On this account a lamp fitted with a bye-pass is greatly 

 to be preferred, so that the light can be turned off between 

 exposures. The larger Perfectol lamps are so fitted, and 



