ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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except at top. As the illustrations show, the fitting socket which is 

 inserted in the stage is sprung, and though the middle passes a conical- 

 shaped pin, to which at the top a little screw-head is attached. By 



Fig. 13. 



Fig. 14. 



screwing on this head the fitting socket is expanded, and hence the butt 

 is held firmly. To release the apparatus the screwing action is reversed. 



Electric Mercury Vapour Lamp for Microscopic Illumination. 

 J. E. Barnard gives the following description of the mercury vapour 

 lamp (fig. 15) exhibited by him on April 17th, 1907. The type of lamp' 

 used for the experiments here described, is that made by the Bastian 

 Mercury Lamp Co. Owing to its convenient size and shape, and small 

 current consumption, it has been found most suitable for microscopical 

 purposes. Owing to the fact that, when 

 mercury vapour is in a condition of in- 

 candescence, the light emitted by it con- 

 sists spectroscopically of bright lines, 

 which are evenly distributed over the 

 visual spectrum, it has therefore been 

 found to have considerable possibilities 

 for microscopic work. 



The Bastian lamp is of the arc lamp 

 type, the light being produced between 

 two bodies of mercury instead of between 

 two carbons. Being inclosed within a 

 sealed glass tube there is no loss of the 

 mercury whatever, and the lamp once set 

 up in operation continues to work with- 

 out adjustment or renewal of any kind, 

 until the " life " of the " burner " por- 

 tion of the lamp is exhausted. This 

 " life " in the nature of things must have 

 some limit, though it is difficult to say 

 at present what that limit is. Probably 

 3000 hours may be regarded as a fair 

 average, though burners have been tested continuously for over 700O 

 hours without any sensible diminution in their efficiency, and it is quite 

 possible that improved methods of manufacture may render a life of 

 6000 hours the rule rather than the exception. 



The lamp as now in use commercially, is, in fact, an arc lamp, that 

 is to say, it is in working much the same as a carbon arc. The differ- 

 ence, however, is that in the mercury lamp the arc itself is very long, 

 and constitutes the source of light. Id the carbon arc this is not the 



Fig. 15. 



