606 PROFESSOR STOKES ON THE LONG SPECTRUM OF ELECTRIC LIGHT. 
The following figure exhibits the principal lines of aluminium, with zinc and cadmium 
for comparison. In the first of the aluminium lines represented, I could not make out 
the division into two parts corresponding to the tips of the electrodes. E, denotes the 
extreme red visible on a screen ; the lines in the visible spectrum are omitted, as this 
Aluminiuia . . . 
Zinc 
Cadmium 
Air 
has been made the subject of elaborate researches by others. The horizontal distances 
are proportional to the distances of the several pricks from that belonging to the extreme 
red, and therefore vary as the chords of the arcs described by the pricker. This tends 
to coiTect to a certain extent the exaggeration of the more refrangible end of the spec- 
trum arising from the mode adopted of laying down the positions of the lines. The 
lowest row of lines in the figure, which is placed here for the sake of comparison, will be 
referred to further on. 
Besides the lens above mentioned, I sometimes employ in a different manner another 
of ^ inch diameter and 2^ inches focal length, and accordingly large for its focal length. 
This is used for forming an image of the spark, which is received on the substance that 
is to be examined, or that is used for examining the spark. The difference of focal 
length for the different rays is so enormous that, while one part of the spectrum is in 
focus, other parts are utterly out of focus, and thus we may judge in a general way of 
the refrangibility of the rays by which any particular effect is produced. In this way 
such concentration of the rays is obtained, that effects may be studied which would not 
bear examination by prismatic analysis. In speaking of this lens I shall call it the 
2‘5-inch lens, from its focal length. 
Absorption of the imisible rays by Alkaloids, Glucosides, &c. 
Before examining these substances it is requisite to dissolve them, and we must fii’st 
inquire into the transparency of the solvent. Fortunately the most useful of all solvents, 
water, is transparent when pure ; and as to reagents, we may employ sulphuric or hydro- 
chloric acid for an acid, these acids being transparent, and ammonia, suppose, for an 
alkali. In speaking of a substance as transparent, I wish it only to be understood that 
it is of a transparency comparable with quartz. As to ammonia, although it absorbs 
the more refrangible rays when in quantity (unless the observed absorption were due to 
some impurity), it may be deemed transparent in the small quantity which alone it is 
Fig. 1. 
