356 Molecules, Ultimates, Atoms^ and Waves. [July, 



diameter of the particles, which would be a little less than 

 double that departure. 



The late Professor W. A. Miller, of King's College, London, 

 in an elaborate paper " On the Photographic Transparency 

 of Various Bodies, and on the Photographic Effects of Metallic 

 and other Spectra, obtained by means of the Electric 

 Spark" ("Philosophical Transactions," 1862, p. 861), has 

 shown how far the actinic effects of the spectrum, obtained 

 from silver electrodes, can be traced beyond the line h. 

 Dividing the portion of the spectrum between b and H into 

 16 equal parts, Professor Miller found that the greatest 

 distance beyond H at which any photographic image could 

 be produced with silver electrodes was 70*5 of these parts, or 

 about 4*4 times the distance between b and H. This result 

 could be obtained only when the light passed through a 

 medium exerting little or no absorptive action on the most 

 refrangible part of the spectrum, such as ice, water, white 

 fluor spar, quartz, atmospheric air, hydrogen, carbonic oxide 

 and carbonic acid gases. In all these media the actinic 

 spectrum extended to the same limit beyond H. With zinc 

 electrodes, however, an image was obtained at a point as far 

 beyond H as 83 of the scale. This may be regarded as the 

 limit of the actinic powers of the spectrum. 



Professor Stokes, again, in a paper "On the Long Spectrum 

 of Electric Light" (" Philosophical Transactions," 1862, 

 p. 599) has shown that, with aluminium electrodes, a line 

 can be traced a good way beyond this limit. What is re- 

 markable, however, is that Professor Miller with aluminium' 

 electrodes could not obtain a photographic image at any 

 point beyond 55 of his scale, much within the limit of the 

 most refrangible zinc line. The very highly refrangible 

 aluminium lines, discovered by Professor Stokes, render their 

 existence manifest only by their effects on a strongly fluores- 

 cent salt of uranium, employed as a screen for their recep- 

 tion — quartz being used for the prism and lens. From a 

 comparison of the diagrams given by Professor Stokes with 

 those given by Professor Miller, it appears that the most highly 

 refrangible wave whose existence is thus manifested by a 

 bright line on the fluorescent screen is very nearly at 113 

 beyond h on Professor Miller's scale, or about 30 beyond 

 the last zinc line. 



No attempt appearsto have been hitherto made to determine 

 by actual measurement the wave-length corresponding to either 

 the extreme zinc or the extreme aluminium line. Our only 

 resource, therefore, is to make a rough estimate, founded on 

 the supposition that the wave-lengths continue to follow a 



