48 THE FAIRY-LAND OF SCIENCE. 
piece of lace. Look what the 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 
FIG. 9. Piece of lace photographed during the lecture. 
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 afterward, otherwise the chemi- 
