sun has been doing while I have been speaking. It has been
breaking up the nitrate of silver on the paper and turning it
into a deep brown substance; only where the threads of the lace
were, and the sun could not touch the nitrate of silver, there
the paper has remained light-coloured, and by this means I have a
beautiful impression of the lace on the paper. I will now dip
the impression into water in which some hyposulphite of soda is
dissolved, and this will "fix" the picture, that is, prevent the
sun acting upon it any more; then the picture will remain
distinct, and I can pass it round to you all. Here, again,
invisible waves have been at work, and this time neither as light
nor as heat, but as chemical agents, and it is these waves which
give us all our beautiful photographs. In any toyshop you can
buy this prepared paper, and set the chemical waves at work to
make pictures. Only you must remember to fix it in the solution
afterwards, otherwise the chemical rays will go on working after
you have taken the lace away, and all the paper will become brown
and your picture will disappear.

And now, tell me, may we not honestly say, that the invisible
waves which make our sunbeams, are wonderful fairy messengers as
they travel eternally and unceasingly across space, never
resting, never tiring in doing the work of our world? Little as
we have been able to learn about them in one short hour, do they
not seem to you worth studying and worth thinking about, as we
look at the beautiful results of their work? The ancient Greeks
worshipped the sun, and condemned to death one of their greatest
philosophers, named Anaxagoras, because he denied that it was a
god. We can scarcely wonder at this when we see what the sun
does for our world; but we know that it is a huge globe made of