PEOrESSOE STOKES ON THE liONO SPECTEUM OF ELECTEIC LIGHT. 603 
The salt is then poured off in suspension from any undissolved crystals of the alkaline 
phosphate employed, and collected on a filter. A pressed cake of this salt, or a porous 
tile on which the salt is spread, having been moistened with a solution of borax, forms 
an admuable screen, and is what I have chiefly employed of late. It shows, of course, 
the visible as well as the invisible rays — the former by ordinary scattering, the latter by 
fluorescence. 
From the circumstances of its formation, the salt is probably (abstraction being made 
of the water of hydration) the original phosphate with the equivalent of constitutional 
water replaced by an equivalent of an alkali, which would make it analogous to the 
highly fluorescent natural yellow uranite. At any rate this hypothesis guides us to its 
successful preparation, the conditions of which it would not have been easy to make out by 
observation alone. Without the use of free acid the fluorescence is not fully developed, 
which is accounted for by the insolubility of the original phosphate and the fluorescent 
salt, which presents an obstacle to the complete conversion of the one into the other. 
Metallic Lines. 
These may be viewed, as already mentioned, by passing the spark of an induction 
coil between two electrodes formed of the metal to be examined (the secondary terminals 
being respectively in connexion with the coatings of a jar of suitable size), forming a pure 
spectrum by a prism and lens of quartz, the faces of the prism being equally inclined to 
the axis of the crystal, and the lens being cut perpendicular to the axis, and receiving 
the spectrum on a suitable screen, for which, if a fluorescent liquid be employed, it is to 
be placed in a quartz-faced vessel, in default of which a piece of filtering paper may 
be saturated with the liquid. 
If the visible spectrum and the very beginning of the invisible be excepted, the lines 
thus seen vary from metal to metal, and therefore are to be referred to the metal and 
not to the air. They are further distinguished from air lines by being formed only at 
an almost insensible distance from the tips of the electrodes, whereas air lines would 
extend right across. The spectrum is far too extended to allow us to regard the whole 
at once as in the position of minimum deviation ; and if the prism be placed at all near 
the electrodes, without which we should have comparatively little light to work with, 
the effect of the different divergency, converted by the lens into convergency, of the rays 
in the primary and secondary planes is very great. In order to obtain a pure spectrum, 
the screen must be in focus as regards the primary plane ; and if a particular point P 
of the spectrum be at a minimum deviation, the lines immediately about P are reduced 
almost to points, which are the images, for light of that refrangibility, of the tips of the 
electrodes, or, to speak more exactly, of the part of the spark just outside the tips. 
But in the secondary plane the rays on one side of P have not yet reached their focus, 
and on the other side have passed it ; so that the image of a point is a line, the primary 
focal line, of a length increasing on receding from P in either direction, and accordingly 
the spectral image of either tip, assumed to be a mere point, would be a pair of slender 
