802 PROFESSOR PIAZZI SMYTH ON COLOUR, 
gray. The reason of the seeming blackness being, apparently, that the depth 
of the blue tint in the pale bottle not being sufficient to reverse the waves and 
convert the blue of the lakey-red of Dr Wi1son’s book into light, the two blues 
add their depths of blue together, making them very dark blue indeed, to a 
mixture of some red, and much dark-greenish shade in the region of the half 
extinguished citron of the spectrum. A species of commingling which cannot 
fail, in a poor light, to produce something very like black. 
Hence the subject of vision of colours through artificial Di-chroic media of 
different depths and different hues,—Judson’s green being merely one among 
many and diverse-tinted colour-media which have more or less of the curious 
property,—seems to promise a wide range of phenomena most interesting to 
Science and possibly, to be, enriching to Art. 
But precisely because the subject is so extensive, my own immediate duties 
require me now to take leave of its strange phenomena; and to trust that 
others better able and more professionally concerned, will continue the inquiry 
and extend the results. 
POSTSCRIPT. 
Colour-perfected Reference-line for Spectroscopic Mensuration. 
All the accuracy of spectroscopic measurement, when performed as it 
generally is by looking into a telescope, depends, very much as in astronomical 
observations, on having good fiducial points of reference in the field of view. 
These fiducial points must be sharp in themselves, in good focus, easily 
seen, and without parallax ; while, for a bright field of observation, they may be 
black-lines or pointers ; and for a dark field, bright ones. The latter are the 
more important in spectroscopy, because, while they may, on a certain con- 
struction, be usable in a bright field also, they alone have any efficiency in a 
dark field ; and every spectrum has dark ends, however bright it may be in the 
middle. 
With this view it was, that after trying several older and less perfect con- 
trivances, I procured three years ago one of the widely praised bright-line refer- 
ences invented and manufactured by Mr Apam Hitcer,* and employed it in the 
observations detailed at the beginning of this paper. Up to a certain extent 
it proved admirable ; for the line of light (formed by a cut through black varnish 
on the face of a fox-wedge of glass dipping into the field of view and illumined 
by a lamp at its outer end) was sharp, easily seen even with the faintest illumina- 
tion-light, and capable of being brought to best focus; while it was also fur- 
* Of 192 Tottenham Court Road, London. 


