346 D/% Darwin’s Experiments on the 
the minute graduation of thermometers* or of clock- faces* 
which are to be feen at a diftance* if the letters or figures are 
coloured with orange* and the ground with indigo ; or the 
letters with red, and the ground with green; or any other 
lucid colour is ufed for the letters, the fpeftrum of which is 
limilar to the colour of the ground; fuch letters will be feen 
much more diftinftly, and with lefs confufion, than in black or 
white : for as the fpeftrum of the letter is the fame colour 
with the ground on which they are feen, the unfteadinefs of 
the eye in long attending to them will not produce coloured 
lines by the edges of the letters, which is the principal caufe of 
their confufion. The beauty of colours lying in vicinity to each 
other, whofe fpedra are thus reciprocally fimilar to each go* 
lour, is owing to this greater eafe that the eye experiences in 
beholding them diftindlly ; and it is probable, in the organ of 
hearing a fimilar circumftance may conftitute the pkafure of 
melody. Sir Isaac Newton obferves, that gold and indigo 
were agreeable when viewed together ; and thinks there may 
be fome analogy between the* fenfations of light and found; 
(Optics, Qu. 14.) 
In viewing the fpe&ra of bright objects, as of an area of red filk 
of half an inch diameter on white paper, it is eafy to magnify it 
to tenfold its fize : for if, when the fpeftrum is formed, you 
ftrll keep your eye fixed on the filk area, and remove it a few 
inches further from you, a green circle is feen round the red 
filk : for the angle now fubtended by the filk is lefs than it 
was when the fpectrum was formed, but that of the Ipeftrum 
continues the fame, and our imagination places them at the 
fame diftance. Thus when you view a fpeftrum on a flieet of 
white paper, if yon approach the paper to the eye, you may 
diminifh it to a point ; and if the paper is made to recede from 
