88 Colours and their Relations. [January, 



food or drink of the bird, have passed through its circulation, 

 and found its ultimate lodgment in those wing-feathers. It 

 is most abundant at the pairing season (see an interesting 

 paper on the subject in "The Student," vol. i., p. 161, where 

 will be found a chromolithograph of the two birds above 

 named). 



All iridescent colours in fibres or spines are adventitious, 

 and belong to the class of colours of thin plates; as, for ex- 

 ample, the beautiful iridescent spines of the sea-mouse 

 (Aphrodite aculeata), and the iridescent branchiae of the Eolis, 

 which serve the double purpose of a breathing apparatus and 

 a bank of oars. The colours of the wings of insects and 

 the elytra of beetles, &c, all fall under the same extensive 

 category. 



The iridescence of mother-of-pearl and the fire of the 

 opal, again, though also phenomena of interference, may 

 perhaps be regarded as rather transitional in their character, 

 approaching towards the colours developed by systems of 

 fine lines, producing the phenomena of diffraction. The 

 simplest case of diffraction is that of the external and in- 

 ternal fringes, developed when a single thin obstacle, such 

 as a fine wire or a very thin opaque plate placed edgewise, 

 is set in the path of a divergent beam of sunlight. In this 

 instance the internal fringes are produced by the overlapping 

 of the waves bent inwards from the opposite sides of 

 the obstacle ; while the outer fringes are due to secondary 

 waves propagated from the outer edges of the obstacle, 

 which interfere with the direct waves coming from the 

 luminous source. It is by a system of extremely fine and 

 very closely approximated equidistant lines that the diffracted 

 spectrum — the purest of all spectra — is produced. 



By far the most beautiful exhibition of adventitious colours 

 is that to be obtained by means of polarised light, or light 

 consisting of waves, the vibrations of which are all per- 

 formed in one plane. To produce the phenomena of colour 

 in this manner, it is needful to have the means of polarising 

 the light in two opposite planes — the plane in which the 

 vibrations are performed in the one set of waves being per- 

 pendicular to that in which they are performed in the other 

 set. The light may be thus polarised either by reflection 

 from a smooth surface at a certain angle, or by means of 

 crystals of Iceland spar, cut so as to form what are called, 

 from their inventor, " Nicol's prisms," or else by means of a 

 plate of tourmaline or of iodide of quinine. Of these 

 appliances one is used for polarising the light, the other as 

 an eye-piece for analysing it, that is to say, for showing 



