OF ORGANIC SUBSTANCES ON THE RAYS OF THE SPECTRUM. 
261 
The Measurement of Bands and tlieir Position. 
The lines of zinc, cadmium, and aluminium oiler a convenient means of determining 
the position of absorption bands, &c., and M. Soret has employed them. The lines 
of cadmium in the ultra-violet as well as in the visible portion of the spectrum 
have been measured by M. Mascart and their wave-lengths calculated. M. Sarasin 
has made use of these measurements in his determination of the refraction indices of 
quartz for the ordinary and extraordinary rays in the ultra-violet spectrum. (‘ Archives 
des Sciences Physiques et Naturelles;’ Geneva, vol. lxi, p. 109; January, 1878.) The 
wave-lengths for the cadmium lines in the invisible portions of the spectrum are shown 
in Diagram No. 1, and it will be seen that several of the following diagrams have been 
made after this scale, it being a,n advantage to have the metallic lines placed at 
intervals corresponding to their wave-lengths. When nickel points instead of cadmium 
have been used, two strongly marked air-lines below the solar line L (Plate 21, 
fig. 1) serve as a starting point for measurements; another air-line between the lines 
11 and 12 also answers the same purpose. Inasmuch, however, as the appearance of 
spectra as seen when photographed is different from that when the lines have been 
mapped out in accordance with their wave-lengths, it has been considered desirable to 
follow the plan of M. Soret and lay down the spectra in some cases just as they appear 
in the photographs. 
Some of the advantages derived from the employment of photography in studying 
the ultra-violet rays are the following : The measurements are free from any personal 
error, the photographs are permanent and unmistakable records easily referred to, and 
the observations are made more rapidly and with much less fatigue to the eye than by 
any other means. Furthermore, liquids which are slightly turbid and unfit for obser¬ 
vation with a fluorescent screen or eye-piece can be examined photographically by 
lengthening the exposure of the plate. 
Absorption Caused by Films of Canada Balsam. 
It was considered very desirable to know whether Canada balsam, as used in the 
construction of optical apparatus, was capable of cutting off the ultra-violet rays to 
any considerable extent. W e are aware, from the researches of M. Soret, that Iceland 
spar is extremely transparent, the 26th line of cadmium being distinguishable through 
a thickness of 65 m.m. of the substance. It was only necessary, then, to take a small 
Nicol’s prism and interpose it between the slit and the first lens. The result showed 
that lenses and prisms, cemented with Canada balsam, are utterly useless for such 
experiments as these, the more refrangible rays being cut off sharply at the 12th line. 
Several kinds of beautifully clear and colourless optical glass, made by Feil, of Paris, 
and lent us by Mr. Hilger, have the same effect upon the spectrum, whether the 
glasses be made of lead or aluminium ; they are all utterly valueless for observations on 
rays more refrangible than line No. 12 of cadmium. Fluor spar is quite transparent. 
