PROFESSOR STOKES ON THE CHANGE OF REFRANGlBILITY OF LIGHT. 561 
artificial discharges, the stronger the spark the more the rays of excessively high refrangibility seem 
to abound, in proportion to the whole radiation. Now a flash of lightning is a discharge incom- 
parably stronger than that of a Leyden jar. It might have been expected, therefore, that the 
radiation from lightning would be found to abound in invisible rays of excessively high refrangibility. 
Yet I could not make out in a satisfactory manner the absorption of the rays by glass, even by 
common window-glass. I do not wish to speak positively regarding the result of this observation, 
for of course observations with lightning are more difficult than those made with a machine which is 
under the control of the observer. Yet it did seem as if the spark from a Leyden jar was richer 
than lightning in rays of so high a refrangibility as to be stopped by glass. If this be really true, 
it must be attributed to one of two things, either the non-production of the rays in the first instance, 
in the case of lightning, or their absorption by the air or clouds in their passage from the place of 
the discharge. If they were not produced, that may be attributed to the rarity of the air at the 
height of the discharge, that is, at the height of the thunder-cloud. No doubt the metallic points 
of the discharger belonging to the electrical apparatus may have had an influence on the nature of 
the spark ; but I am inclined to think that this influence, so far as it went, would have acted in the 
wrong direction, that is, would have tended to produce rays of lower, at the expense of those of 
higher refrangibility. 
Note K. Art. 220. 
My attention has recently been called to a paper by M. Brucke (Poggendorff’s Annalen, B. v. 
(1845) S. 593), in which he describes some experiments which show that the different parts of the 
eye, and especially the crystalline lens, are far from transparent with respect to the rays of high 
refrangibility. The eyes employed were those of oxen and some other animals ; and the inquiry 
was carried on by means of the effect which hght that had passed through the part of the eye to be 
examined produced on a film of tincture of guaiacum that had been dried in the dark. Of course 
the phenomena described in the present paper afford peculiar facilities for such an inquiry, and I 
had frequently thought of entering upon it, but have not yet made any observations. Independently 
of the facility of the observations, and the advantage of being able to examine readily light of each 
degree of refrangibility in particular, the results obtained by means of sensitive media seem to be 
more trustworthy on this account, that it would be possible to employ fresh eyes. The experiments 
of M. Brucke necessarily occupied a considerable time, and it may be doubted whether the eye, 
especially after dissection, might not have changed in the interval, and whether the results so 
obtained are applicable to the eye as it exists in the living animal. 
