If, when the sun shines brightly, a piece of paper be held in the
focus of the rays drawn by the burning-glass, it will take fire. This
experiment succeeds best with brown or any dark-coloured paper: for,
though the glass will collect an equal number of rays upon white as
upon coloured paper, the white paper reflects the rays instead of
allowing them to enter it; hence, the white is not so soon burnt as
the coloured paper, which absorbing more light than it reflects, soon
becomes heated and takes fire.