PROCEEDINGS OF THE SOCIETY. 413 



Mr. Tavemer said he agreed with the first speaker, although at first 

 he had had a difficulty with some of the IS ernst lamps ; but on returning 

 them to the makers, it was found that the trouble was due to their 

 having been incorrectly marked, and, with the exception of the first, all 

 had gone very well. He bad only given up the Nernst, because he had 

 adopted an arc light for a stronger light. 



Mr. J. Hopkinson recommended incandescent gas as giving a brilliant 

 white light, the intensity of which could be easily modified to any degree 

 without moving the lamp. 



Mr. J. Rheinberg said that a very interesting subject that might be 

 discussed in regard to this lamp was as to its suitability for critical 

 images. It was commonly supposed that the different points of a flame 

 bore no phase-relation to one another, and this was supposed, more 

 especially by one school of writers on the theory of microscopic 

 vision — the one to which he believed Mr. Gordon adhered — to be of 

 special benefit as regards critical images. But in the disk source of 

 light which this lamp presented as the source of illumination for the 

 Microscope, all the points were in phase-relation to one another. He 

 himself (i.e. Mr. Rheinberg) saw no objection in that, as he was not 

 an adherent of the views above mentioned, and he thought the new 

 lamp would be quite as suitable for critical images as a flame source, 

 from the theoretical point of view. He would be interested to hear 

 what Mr. Gordon had to say to the point raised. 



The Chairman remarked that he, too, had a favourite light to recom- 

 mend. Some time ago, before the advent of the electric light, public- 

 houses were largely lit up by what was called the albo-carbon gas light, 

 a light obtained by first passing ordinary coal-gas, as a preliminary to 

 combustion, through a vessel containing heated naphthalene. The 

 flame was a large and brilliantly white one. 



Mr. Tavemer inquired how the lamp could be regulated. 



The Chairman said this could be done by cutting off a portion of the 

 light with a metal plate with a circular hole in it. In this way a sharply 

 defined circular source could in effect be obtained. 



Mr. Gordon said he should like to say a few words in reply to the 

 observations which had been made. With regard to the remarks of 

 Mr. Barnard and Dr. Spitta, there was, perhaps, not so much conflict as 

 there seemed to be, as it appeared that under certain conditions Nernst 

 lamps worked well, but there were some conditions under which they did 

 not. He had used them on a voltage of 105, and his was also an alter- 

 nating current ; but Mr. Barnard worked with much higher voltage, and 

 possibly on a direct current — this would, of course, be much more favour- 

 able to the lamp. His own experience had been much the same as Dr. 

 Spitta's. Mr. Hopkinson had a weakness for the incandescent gas 

 lamp, the objection to which was that they could not get an image of the 

 light-source without getting an image of the mesh of the mantle, unless 

 ground-glass was used, or the source was thrown out of focus. If a 

 Welsbach mantle was thrown out of focus, its image appeared in the 

 Ramsden circle, and formed there the optical equivalent of a thread-like 

 aperture, and giving rise to a very unpleasant amount of diffraction ; 

 and under these conditions this kind of lamp impaired all fine detail in 



