252 Prof. G. D. Liveing on the TJltra-Violet Spectra. [March 9, 



a succession of nine shaded bands at nearly equal intervals between 

 M and S, while in the highest regions of the spectrum it seems to be 

 quite transparent. 



I mentioned at the outset the probable connection between the 

 intensity of the solar radiation and the sensitiveness of our eyes to 

 rays of different colours. The consideration of ultra-violet absorption 

 spectra leads to the mention of another fact connected with vision, or 

 rather with the construction of the eyes of the higher animals. Soret 

 has investigated, and recently Chardonnet has more fully examined, 

 the limits of transparency of the crystalline, cornea, and vitreous 

 humour of the eyes of various animals and man, and found them all 

 more or less transparent for ultra-violet rays. The limit of trans- 

 parency in many cases approaches, but never exceeds, the limit of the 

 solar spectrum. Chardonnet places the limit of transparency of the 

 crystalline of the human eye as low as M, which is not consistent 

 with the observations of Herschel and Helmholtz before mentioned, 

 but this inconsistency is probably due to alterations which had taken 

 place after death in the eyes experimented on by Chardonnet. That 

 the transparency of the materials of the eye does not extend beyond 

 the solar line U, Chardonnet regards as a provision of nature to 

 protect the retina from the extreme radiations of artificial lights ; but 

 I venture to offer a different explanation, which is, that the selection 

 of the materials of the eye has been determined not by what they 

 will absorb but by what they will transmit. If the materials in 

 question w^ere in any great degree opaque to the ultra-violet solar 

 rays, these rays must be absorbed and must either be used in heating 

 the absorbent or do work upon it in some form, perhaps alter it 

 chemically, and so impair its efficiency as part of an optical instru- 

 ment. I see, then, in the selection of these materials for our eyes 

 an instance, one amongst many, of the marvellous adaptation of our 

 organisation to the natural, rather than to the artificial surroundings 

 in which we are placed. [G. D. L.] 



DESCRIPTION OF THE PLATES. 



In the photographic plate : — 



No. 1 shows en expansion of the magnesium line at wave-length 2852, while 

 it is also so strong. y reversed as to produce a complete obliteration of all the lines 

 above U within tue range of the photograph, while its bright wing reverses the 

 iron lines near T. 



No. 2 shows the same line at 6 mych less expanded but still self- reversing. 

 The lines at a are also magnesium lines, wave-lengths 2795 and 2801. Most of 

 the lines between b and S are chromium lines. 



No. 3 shows iron lines reversed by putting iron wire into the arc. 



No. 4 shows calcium lines in recurring triplets ; also the cyanogen bands 

 between K and M and at N. 



No. 5 shows three of the zinc triplets. 



No. 6 is the brightest part of the ultra violet spectrum of water. 



No. 7 shows part of the channelled spectrum of nitrogen in the ultra violet. 



The lithographic plate gives the position of the ultra violet lines of several 

 metals to a scale of wave lengths. 



