an body
resembling the sulphate of quinine, which causes all the tissues of
the body to be more or less fluorescent. All animal infusions show
this fluorescence. The crystalline lens of the eye exhibits the effect
in a very striking manner. When, for example, I plunge my eye into
this violet beam, I am conscious of a whitish-blue shimmer filling the
space before me. This is caused by fluorescent light generated in the
eye itself. Looked at from without, the crystalline lens at the same
time is seen to gleam vividly.

Long before its physical origin was understood this fluorescent light
attracted attention. Boyle describes it with great fulness and
exactness. 'We have sometimes,' he says, 'found in the shops of our
druggists certain wood which is there called _Lignum Nephriticum,_
because the inhabitants of the country where it grows are wont to use
the infusion of it, made in fair water, against the stone in the
kidneys. This wood may afford us an experiment which, besides the
singularity of it, may give no small assistance to an attentive
considerer towards the detection of the nature of colours. Take
_Lignum, Nephriticum_, and with a knife cut it into thin slices: put
about a handful of these slices into two or three or four pounds of
the purest spring water. Decant this impregnated water into a glass
phial; and if you hold it directly between the light and your eye, you
shall see it wholly tinted with an almost golden colour. But if you
hold this phial from the light, so that your eye be placed betwixt the
window and the phial, the liquid will appear of a deep and lovely
ceruleous colour.'

'These,' he continues, 'and other phenomena which I have observed in
this delightful experiment, divers of my friends have looked upon, not
without some wonder; and I remember an excellent oculist, finding by
accident in a friend's chamber a phial full of this liquor, which I
had given that friend, and having never heard anything of the
experiment, nor having anybody near him who could tell him what this
strange liquor might be, was a great while apprehensive, as he
presently afterwards told me, that some strange new distemper was
invading his eyes. And I confess that the unusualness of the
phenomenon made me very solicitous to find out the cause of this
experiment; and though I am far from pretending to have found it, yet
my enquiries have, I suppose, enabled me to give such hints as may
lead your greater sagacity to the disco