New Method of Using the Electric Arc. By E. B. Stringer. 279 



then be effected visually by means of the former, leaving the finer 

 detail to be resolved photographically by the shorter waves. Much 

 assistance might also be got by the use of a fluorescent screen, 

 though I have hitherto tried this method without much success. 

 A transparent fluorescent screen such as is used in spectroscopic 

 work must be inclined to the optic axis, and a focussing screen, by 

 the nature of the case, cannot be so inclined. I have also tried an 

 opaque fluorescent screen, the platinocyanide one, observing the 

 image upon its surface through the door in the side of the camera ; 

 but it is exceedingly difficult to see fine detail in this way, though 

 the screen becomes strongly fluorescent, and the decided grain of 

 the screen is also a hindrance. 



Such experiments can, however, be only very imperfectly made 

 with the lenses which are at present available ; though the fact 

 that the screen does become fluorescent, proves that they transmit 

 the invisible rays to a considerable degree. This is also evident 

 from the fact that they give a badly defined photographic image 

 without the quinine solution ; and that if the ammonia-copper 

 solution (which is partly opaque to the most refrangible rays 

 transmitted by glass) be also removed, the definition is still more 

 imperfect. 



This is shown in the three photographs of Navicida Bonibus. 



(A) is with the light from the arc, the ammonia-copper and 

 quinine solutions being both used. 



(B) is the same without the quinine solution. 



(C) is the same without either the copper or quinine. 



The images in all of them were focused visually with equal 

 sharpness. 



I find that Stokes used in his investigations a fluorescent 

 screen of uranium phosphate, which might prove to be better 

 than barium platinocyanide for the present purpose. By means 

 of it he discovered that solutions of the vegetable alkaloids 

 exercise a powerful selective absolution in the ultra-violet ; so 

 that by a suitable choice of these it might be possible to make the 

 ultra-violet rays also " monochromatic " ; especially as the banded 

 character of the arc spectrum is no doubt continued throughout 

 that region. All this might some day be done. But the difficulties 

 to be overcome are so many that I confess I submit these con- 

 siderations to the Society with a good deal of diffidence, having 

 much doubt as to whether they will ever prove to be of any 

 practical value. 



I have since found that the Microscope (and the same lenses 

 with which the photographs were clone) transmits both ultra- 

 violet bands, having succeeded in projecting them upon the fluores- 

 cent screen by means of a slit, lens, and prism, placed beyond the 

 eye-piece. The second or more refrangible one, however, appears 



