IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 791 
(2.) DOUBLE-SPECTRUM COLOURS. 
Nunber. Name. Spectral Region. 
16 Amaranth. Compound of 36 000 and 59 000 variously. 
17 Rose. ditto. 
18 Lilac. ditto. 
19 Purple. ditto. 
20 Azure. ditto. 
(3.) TRIPLE-SPECTRUM COLOURS. 
Number. Name. Spectral Region. 


Very compound; of 36 000, 46 000, and 
aS ed ory 59 000 variously. 
22 Yellow-gray. ditto. 
23 Green-gray. ditto. 
24 Blue-gray. ditto. 
25 Black-gray. ditto. 


Throughout the series of 25 colours thus named, there should be one or two 
intermediate colour gradations easily imaginable between every pair. While of 
each of them again, as also of the previous ones, there are three or more degrees 
of lighter, or mixed with white ; and three or more degrees of darker, or mixed 
with black (see Plate 3), easily realisable; and making in all about 400 
practical tints for double-star observers to sharpen and define their colour senses 
upon; so long as they think it right, or expedient to confine themselves to 
eye-observations alone, touching star-colour, its causes and effects. (See p. 843.) 
LIMITS OF NEWTON'S “ Nee variat lux fracta colorem.” 
To make violet, the artists tell us to mix red and blue; and to make laven- 
der, add a little yellow to the previous mixture; and we may thereby obtain 
something which will pass muster with the eye alone, but not with the spectro- 
scope, nor with photographic preparations. And inasmuch as blue is already 
lower in refrangibility than either violet or lavender, the addition of red or 
yellow or both can only make it lower than ever; nor is there any known 
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