I'M:)'] on Applications of Polarized Light 729 



Phys. 1903 (IV), Vol. XII. 115o ; Sclioiirock, Zeit. Vereins Deutscb. 

 Zuck. Ind., Tech. Part, 1903, Vol. LIII. G.")2). Through the personal 

 kindness of Mr. Biistian, 1 was enabled about three years later to 

 make use of the same source of light in what is still, perhaps, its 

 most convenient form. The glass liastian lamp was designed to 

 burn with the coils of the arc in a horiz.ontal plane, and was arranged 

 to light automatically in this position. It was with great delight, 

 therefore, that I discovered that, in spite of all warnings to the 

 contrary, the lamp would continue to burn for any length of time 

 with the coils raised into a vertical plane ; in this position one of the 

 straight portions of the arc could be focused by a condenser directly 

 on to the slit of a spectroscope, and so used to illuminate the field of a 

 polarimeter. The lamp consumed very little current, and could be con- 

 nected directly to the ordinary lighting circuits without any risk of 

 ■" blowing " the fuses ; it was cheap to purchase, and as the resistances 

 formed part of the holder of the lamp there was no need for any 

 auxiliary apparatus whatever. In view of its special suitability for 

 polarimetric work, it is to me personally a matter of some regret that 

 this pioneer lamp has been displaced completely by the more powerful 

 arcs, encased in refractory silica glass, which now adorn the exteriors 

 of so many places of amusement. I use the word " exteriors " 

 advisedly. Dr. Gladstone in 18(50, in describing the properties of 

 the mercury light, " was struck by the strange manner in which it 

 modified the apparent colours of surrounding objects, and especially 

 with the ghastly purple and green hues which it imparted to the 

 faces and hands of the spectators.'" These green and purple hues 

 would not be likely to find favour with a modern audience, and it is 

 therefore not surprising that this powerful and economical lamp is 

 more valued as a source of light for factories and dockyards than 

 for exclusive use in lecture theatres and drawing-rooms. 



AVhen using the mercury arc as a source of violet light, account 

 must l)e taken of the greatly reduced sensitiveness of the eye to 

 light of such short wave-length. It is here that the silica mercury 

 lamp has proved of such great utility. I am indebted both to 

 Mr. Lacell, of the Silica Syndicate, and to the Brush Electrical 

 Engineering Company for allowing me, for experimental purposes, 

 to distort their well-considered designs for commercial mercury arc 

 lamps. Here, for instance, is a horizontal lamp which has been 

 altered so that the arc can be seen at its greatest intensity in an end- 

 on position. At first the light was liable to be obscured by globules 

 of condensed mercury. But by recessing the window it was kept 

 sufficiently hot to prevent condensation, and this difficulty was 

 effectively overcome. Even then, however, the arc was not so con- 

 venient as one arranged in a vertical plane, like the upturned Bastian 

 lamp. It was at this stage that I persuaded the Brush Company to 

 modify for me their '" Quartzlite " lamp by twisting one of the ter- 

 minal U -tubes into such a position that it did not empty itself when 



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