( 779) 
XXXIII.— Colour, in Practical Astronomy, Spectroscopically Examined. 
By Prazzai Smytu, Astronomer Royal for Scotland. 
(Plates XLI-—XLIIT.) 
(Received May 24th, 1878.) 
CONTENTS. 
: PAGE PAGE 
THE APOLOGY, . g 4 ces CxHEmiIcaL Inpication By D1-CHRoICcs, . 795 
CoLouRED Guass SuRrEs, ; : a) 70 SUBJECTIVE CONDITION OF THE COLOUR- 
CoLOURED FLUID SERIES, : 782 BLIND, SO-CALLED, . : . 796 
First Resutt, from both Glasses and Fluids, . 782 Tuer Art-PROBLEM, GREEN, . ; », » 199 
SECOND RESULS, as to certain Laws, . 783 POSTSCRIPT, a coloration perfected, . m c02 
PHYSICAL, AS AGAINST OPTICAL, ARRANGE- ae pir 
MENT OF CoLoUR MEDIA, . ‘ . 784 ‘ 
PRACTICAL APPLICATION IN SPROTROSCOPY, a aks AESENDES IL Seca Ses reo ns, Ue 
i II.—Coloured Fluid Observations,. 816 
NoMENCLATURE OF CoLouRS OPTICALLY, for 
Double-Star Observers chiefly, . . 789 Pate 1, (41 of Vol.)—Coloured GlassSeries,. 842 
Luuits or Newton’s “Nec Varrat Lux » 2, (42 of Vol.)—Coloured Fluid Series,. 842 
Fracta CoLorEM,” 791 > 3, (43 of Vol.)\—Nomenclature of Colours, 843 
VISION OF COLOURS THROUGH CoLourED (The Plates are also mentioned on pp. 781, 782, and 
Menta, both Mono-Chroic and Di-Chroic, 793 789 to 791.) 

THe APOLOGY. 
Colours and colour sensations are much referred to still by double-star 
observers, who make use of a most extensive range of names, and differ widely 
from each other, in describing the many different tints which they claim to 
recognise by simple eye-observation in those interesting objects. 
Material coloured shades, generally in glass, are also of daily use in both 
terrestrial and nautical astronomy, in decreasing or qualifying the intensity of 
sun’s, moon’s, and other rays; while special varieties of such media are still 
being inquired after for acting, if possible, with improvement of, rather than 
detriment to, the nicest definition of the optical instruments concerned. 
And again coloured glasses have been latterly demanded, which should 
allow only the strictest mono-chromatic light to pass through them in one or 
another particular grade of the spectrum,—so as to allow, as one example only, 
the solar red-prominences to be plainly and broadly observed, without the 
usual spectroscopic draw-back of having to look at them through the narrowest 
of slits. ‘ 
Inasmuch as fully successful results have hardly yet been obtained by the 
world in all these lines of inquiry; and as the best hitherto published data of 
experiment, so far as known to me, have not taken up several points of sensible 
importance to practical men,—I have employed a few weeks of enforced leisure 
in the beginning of this year, 1878,—in completing certain spectroscopic exa- 
VOL, XXVIII. PART III. 9s 
