
780 PROFESSOR PIAZZI SMYTH ON COLOUR, 
minations of colours commenced in 1876, with the hope of supplying some of 
the indicated desiderata. 
These new examinations were commenced under the form of observing 
‘‘ absorption-spectra;” a well known branch already of spectrum analysis, but 
applied now with some further precautions upon any, or all, of the signally 
colouring materials which I could easily obtain; observing each of them 
separately under several different thicknesses; and ultimately recording the 
features so made visible both in wave-number spectrum place; and, though 
roughly, in intensity and quantity as well. 
The instrument used was my private “ Aurora Spectroscope;” and from its 
series of several available powers of dispersion, I chose, as most suitable to ) 
the generally ill-defined visual phenomena concerned, that of only 1°: 2 between 
A and H. It was given by a prism of very white flint glass constructed by 
M. Satizron of Paris, of large size but small refracting angle, viz., 30°. The 
objectives of both collimator and telescope were 2°3 inches in diameter, and 
the magnifying power of the latter, armed with Mr Hutcer’s bright-line- 4 
reference* eye-piece, carrying quartz lenses, was ten times linear. The micro- 
meter measurements were made by moving the whole telescope angularly by 
a large tangent screw, the divisions of whose head were afterwards reduced 
to wave-numbers by reference to ANcsTRom’s Normal Solar Spectrum. 
More peculiarly still for the practical object in view, the coloured media 
were not placed immediately in front of the slit, but in an anterior arrangement 
of object-glasses which allowed 4 or 5 square inches of the medium, either in 
glass plates, or flat glass bottles, to be utilised. The slit itself, though rather 
wide, was yet only so moderately opened, that something could still be seen by 
its means, of the chief Fraunhofer lines in the day-light sky, if looked for. But 
the ultimate observations were always made at night, upon the foundation 
offered by the continuous spectrum of a bright coal-gas flame, as duly chronicled 
at the head of every list of numerical observations. 
? 
COLOURED GLASS SERIES. (See also Plate 1, No. 41 of this Vol.) 
Coloured glasses were first examined, and were taken somewhat in spectrum 
order of colour, beginning with the ultra-red; so that they come 
RED-LILAC, 
RUBY-RED, 
YELLOW-BROWN, 
GREEN, 
COBALT-BLUE, and. 
BLUE-LILAC. 
- This neat little contrivance, admirable up to a certain extent, was yet found to have a weakness 
in certain parts of the Spectrum, the nature, and subsequent complete cure of which will be found 
cescribed in the “ Postscript” to this paper, at pp. 802-804. 
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