PEOrESSOE ST0E3]S ON THE LONO SPECTEIJM OE ELECTEIC LIGHT. 605 
the increasing refraction by the lens of rays of increasing refrangibility, the locus of the 
foci of the different rays formed an arc of a curve, or nearly a straight line, lying very 
obliquely to the axes of the pencils coming through the lens. The projection of this 
line on the board having been marked, a line was drawn bisecting this at right angles, 
and at a point in the latter line situated Ilf inches from the former*, the board was 
pierced for the insertion of a pivot, which carried two wooden rulers, which could be 
clamped together at any convenient angle. The shorter of these carried a vertical 
needle, which as the ruler was turned moved in front of the focus of the different rays 
at the distance of about a quarter of an inch. The longer ruler carried a pricker, 
destined to mark on a sheet of paper, temporarily fastened to the drawing-board, the 
position of any object observed. Thus the prism, the lens, the axis of motion of the 
needle and pricker, and the pin for fixing the angle of incidence retained an invariable 
relative position when the drawing-board was moved. In observing, the electrodes were 
placed at the proper distance, and the board turned till the edge of the shadow fell on 
the pin. The rulers were then turned together till any bright line or other object was 
eclipsed by the needle, and its place was then pricked down. To obtain a fixed point 
of reference, I generally pricked down the position of the extreme red visible on a screen, 
such as a piece of paper ; but if great accuracy were required, it might be better to 
employ a well-marked green air line. 
The metals the spectra of which I have observed are Platinum, Palladium, Gold, 
Silver, Mercury, Antimony, Bismuth, Copper, Lead, Tin, Nickel, Cobalt, Iron, Cadmium, 
Zinc, Aluminium, Magnesium. Several of these show invisible lines of extraordinary 
strength, which is especially the case vdth zinc, cadmium, magnesium, aluminium, and 
lead, which last, in a spectrum not generally remarkable, contains one line surpassing 
perhaps all the other metals. Other metals exhibit lines which in certain parts of the 
spectrum are both bright and numerous ; so that, in taking a rough view of the whole, 
certain parts of the spectrum are bright and tolerably continuous, while other parts are 
comparatively weak. This grouping of the lines is especially remarkable in copper, 
nickel, cobalt, iron, and tin. Of the metals mentioned, magnesium gives by far the 
shortest spectrum, ending in a very bright line, beyond which, however, excessively 
faint light may be perceived to a distance about as great as the extent of the longer 
spectra. Aluminium, on the other hand, stands at the head of the above metals for 
richness in rays of the very highest refrangibility ; and it is to this part of the spectrum 
that the strong lines above mentioned belong. In calling these lines strong, it must be 
understood that some allowance is made for their very high refrangibility ; for when 
observed as above described they do not appear absolutely quite so strong as the bold 
lines of zinc or cadmium. This is partly due to the defective transparency of quartz, 
which for this part of the spectrum shows itself by no means perfect ; and indeed the 
highest aluminium line, which is a double line, can only be seen by rays which pass 
through the prism near its edge. 
* A longer distance would have been better. 
