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microscopical ; but as it has several points of great novelty whicb 

 make it extremely useful for this purpose, it was thought to be 

 of sufficient interest to show to the Club. The usual vapour 

 lamp is arranged for running on a 200-volt circuit, and takes 

 the form of a luminous cylinder about 7 ft. in length, giving 

 a great total luminosity with a relatively low specific intensity, 

 which renders it unsuitable for microscopical purposes, and 

 particularly for photomicrography. In the new lamp the un- 

 wieldy length of nearly 7 ft. has dwindled to about 4 in., 

 although the candle-power remains the same about 3,000 

 and the gain in the specific intensity is so enormous as to render 

 it eminently suitable for microscopic use. The secret of the 

 great reduction in length lies in the use of fused quartz in the 

 manufacture of the tube. Quartz is a peculiar mineral in many 

 ways, one among which is that it is exceedingly hard ; it requires 

 a very high temperature to fuse it, it is transparent to the ultra- 

 violet rays, and it has an exceedingly small coefiicient of expan- 

 sion. This, in practice, means that the quartz tube can be run 

 at a temperature which would soften a glass tube, when the sides 

 would immediately collapse through air pressure. The very 

 small coefiicient of expansion prevents any cracking of the 

 tubes through heat, and it is stated that they could be thrown 

 while red-hot into a bucket of water without any risk of 

 fracture. The lamp requires a current of 3^ amperes. A 200- 

 volt tube will accordingly burn nearly three-quarters of a unit 

 in an hour, A word of caution is necessary with i-egard to the 

 ultra-violet rays, in which this lamp is extremely rich. The 

 radiation is so powerful that it will sterilise a Petri dish culti- 

 vation in less than a minute. It is the composition of the light 

 itself which renders it so useful in microscopy. Used without 

 a filter, the absence of red rays improves the performance of 

 an indifferently corrected objective. The peculiar mercury 

 spectrum gives a unique opportunity of easily obtaining strictly 

 monochromatic light in large quantity, with a choice of several 

 wave-lengths. There are six principal lines in the spectrum- 

 two in the orange, one in the green, one in the blue and two 

 in the violet. The green line A. 5461 is perhaps the most useful, 

 and forms at present the most powerful source of monochromatic 

 light available. Mr. Banfield suggested that it would be worth 

 while for opticians to construct a special set of objectives in 



