86 Colours and their Relations. [January, 



beauty in the north of Scotland, and he still retains a lively 

 recollection of its aspect. The phenomenon varies consi- 

 derably, but in general it may be said to consist in the for- 

 mation of several large luminous rings or arches, sometimes 

 coloured, sometimes only bright, at some distance from the 

 sun or moon, and intersecting each other at two or more 

 points — the points of intersection being usually occupied by 

 the mock-sun or mock-moon. Sometimes there are only two 

 of these spectral images of the luminary — one on either side 

 of the true disc, and at a considerable distance from it. In 

 other instances there are three or four — more rarely six such 

 spectral images. To this latter category belonged the remark- 

 able parhelion seen by Scheiner in 1630, of which a parti- 

 cular description was handed down by Gassendi, the astrono- 

 mer. See the " Edinburgh Encyclopaedia," vol. x., p. 613, 

 where several other forms of the phenomenon are described. 

 In that of 1630 there was one complete luminous ring 

 around the sun, another much larger passing through the 

 disc of the luminary, a third of nearly the same size sur- 

 rounding the sun, but of which the lower third was invisible; 

 while there was a portion of a fourth touching the upper 

 limit of the third, and stretching thence upwards a short 

 way towards the zenith. Of the spectral images of the 

 sun, four were situated in the large ring passing through 

 his true disc. They were formed at the points where this 

 ring was intersected by the other two, which had the true 

 sun for their centre. The fifth image was situated right 

 over the true sun, on the margin of the innermost of those 

 two surrounding rings; while the sixth was situated also right 

 above the true sun at double the distance from his disc, on 

 the margin of the second surrounding ring, at the point 

 where it was cut by the fragmentary ring at its summit. 

 The spectral images seen by Scheiner continued visible for 

 upwards of four hours. 



The second case of adventitious colour is that due to the 

 interference of one luminous wave with another — the two 

 proceeding from very closely approximated surfaces. The 

 system of rings, named after their discoverer, Sir Isaac 

 Newton, presents this phenomenon in its simplest form. 

 To obtain these in perfection, it is necessary to place a long 

 focused convex lens against a little longer focused concave 

 lens, and to exhaust the air from between them, so as to 

 press them very closely and equably together by atmospheric 

 pressure. The colours of the reflected and transmitted 

 light are in every case complementary to each other, being 

 such as would, by their union, produce white light. When 



