appearing as the result of the mixture of the blue and yellow pigments, is obtained by the subtraction of colours; it is due to the absorption, by the blue and yellow pigments, of all the spectrum, practically, except the green portion.  In the case of coloured objects, we are then confronted with the fact that these objects appear coloured because of an absorption by the colouring matter of every part of the rays of light falling thereupon, except that of the colour of the object, which colour is thrown off or reflected.  This will appear clearer as we proceed.  Now let me point out a further fact and indicate another step which will show you the value of such knowledge as this if properly applied.  I said that if we selected from the coloured light spectrum, separated from white light by a prism, say, the orange portion, and boring a hole in our screen, if we caught that orange light in another prism, it would emerge as orange light, and suffer no further analysis.  It cannot be resolved into red and yellow, as some might have supposed, it is monochromatic light, i.e. light purely of one colour.  But when a mixture of red and yellow light, which means, of course, a mixture of rays of greater and less refrangibility respectively than our spectral orange, the monochromatic orange—­is allowed to strike the eye, then we have again the impression of orange.  How are we to distinguish a pure and monochromatic orange colour from a colour produced by a mixture of red and yellow?  In short, how are we to distinguish whether colours are homogeneous or mixed?  The answer is, that this can only be done by the prism, apart from chemical analysis or testing of the substances.

[Illustration:  FIG. 16.]

The spectroscope is a convenient prism-arrangement, such that the analytical effect produced by that prism is looked at through a telescope, and the light that falls on the prism is carefully preserved from other light by passing it along a tube after only admitting a small quantity through a regulated slit.

Now all solid and liquid bodies when raised to a white heat give a continuous spectrum, one like the prismatic band already described, and one not interrupted by any dark lines or bands.  The rays emitted from the white-hot substance of the sun have to pass, before reaching our earth, through the sun’s atmosphere, and since the light emitted from any incandescent body is absorbed on passing through the vapour of that substance, and since the sun is surrounded by such an atmosphere of the vapours of various metals and substances, hence we have, on examining the sun’s spectrum, instead of coloured bands or lines only, many dark ones amongst them, which are called Fraunhofer’s lines.  Ordinary incandescent vapours from highly heated substances give discontinuous spectra, i.e. spectra in which the rays of coloured light are quite limited, and they appear in the spectroscope only as lines of the breadth of the slit.  These are called