94 PRACTICAL PHOTO-MICROGEAPHY 



start at the first tilt, and this may have to be repeated two or 

 three times. To do away with this disadvantage Messrs. 

 Baker now arrange that by depressing a knob, on the right- 

 hand side, some portion of the resistance is cut out. The 

 lamp is then allowed to run for half a minute or so before 

 the knob is released, and by this means it will be found that 

 there is usually no difficulty in getting it to light even after 

 tilting it once only. 



It is essential in connecting it up to any electrical supply 

 that its polarity should be strictly observed. The negative 

 wire should be connected to the end of the tube which has the 

 smaller bulb (shown in the illustration on .the left-hand side), 

 and the positive pole to the opposite end. 



Should the surface of the mercury appear to boil as it re- 

 treats along the tube, the connections are not as they should 

 be, and the plug should be reversed. 



When the lamp is running, slight ebullition should be 

 apparent at the negative pole. 



It will be found that for visual work the intensity of illu- 

 mination is sufficient for use with the highest powers. But for 

 photographic work where greater luminosity is required, tubes 

 can now be obtained of quartz or fused silica. They are 

 practically of the same shape and dimensions as the glass 

 tubes ; it is possible to run a much heavier current through 

 them, and with a higher current- density the illuminating 

 power rapidly increases. With a current of three amperes, the 

 lamp is sufficiently powerful for high-power work, and the 

 exposures compare favourably in duration with, for instance, 

 the blow- through lime-light jet. 



The quartz tubes are, unfortunately, much more expensive 

 than the glass ones. Were it otherwise there would be no 

 object in using glass at all, for it would be entirely superseded 

 by quartz. Both types of tube are made by the Bastian 

 Mercury Lamp Company, or by their licensees, and can be 

 obtained from Messrs. Baker. 



A further modification of the mercury-vapour lamp has 

 quite recently been brought out by Messrs. Swift & Son (Fig. 28). 

 This does not differ in principle from the form already described. 

 The mercury-vapour tube itself is carried in a metal box, in the 

 front of which is an iris-diaphragm, so that the available size 



