so to give an account of the other
phenomena exhibited by the selenite when the analyzer is turned
round, viz. that when the angle of turning amounts to 45 deg., all trace
of colour disappears; and also that when the angle amounts to 90 deg.,
colour reappears, not, however, the original colour, but one
complementary to it.

'You see in the spectrum of the reddish light produced by the
selenite a broad but dark band in the blue; when the analyzer is
turned round the band becomes less and less dark, until when the
angle of turning amounts to 45 deg. it has entirely disappeared. At this
stage each part of the spectrum has its own proportional intensity,
and the whole produces the colourless image seen without the
spectroscope. Lastly, as the turning of the analyzer is continued, a
dark band appears in the red, the part of the spectrum complementary
to that occupied by the first band; and the darkness is most
complete when the turning amounts to 90 deg.. Thus we have from the
spectroscope a complete account of what has taken place to produce
the original colour and its changes.

'It is further well known that the colour produced by a selenite, or
other crystal plate, is dependent upon the thickness of the plate.
And, in fact, if a series of plates be taken, giving different
colours, their spectra are found to show bands arranged in different
positions. The thinner plates show bands in the parts of the
spectrum nearest to the violet, where the waves are shorter, and
consequently give rise to redder colours; while the thicker show
bands nearer to the red, where the waves are longer and consequently
supply bluer tints.

'When the thickness of the plate is continually increased, so that
the colour produced has gone through the complete cycle of the
spectrum, a further increase of thickness causes a reproduction of
the colours in the same order; but it will be noticed that at each
recurrence of the cycle the tints become paler, until when a number
of cycles have been performed, and the thickness of the plate is
considerable, all trace of colour is lost. Let us now take a series
of plates, the first two of which, as you see, give colours; with
the others which are successively of greater thickness the tints are
so feeble that they can scarcely be distinguished. The spectrum of
the