226 HAEMOGLOBIN 
propose that the band in the extreme violet should henceforward be 
distinguished as the band y, or the band of Soret, in the spectrum of 
oxyhemoglobin. 
Methods of demonstrating the hand of Soret. — The limits of visibility 
of the solar spectrum correspond, as usually stated, with the H group of 
lines; here lies the arbitrary boundary which separates the extreme 
violet from the ultra-violet properly so called — that region which 
we can only see by interposing fluorescent media in the path of the 
rays (e.g. a fluorescent eyepiece), or by allowing the spectrum to fall 
on a fluorescent surface — the region which is best studied by the aid of 
photography. 
Although Soret's band lies at the limit, but yet within the boundaries, 
of the visible spectrum, it is impossible to see it with the ordinary 
spectroscope, i.e., unless this be provided with special devices. It has 
already been stated that it can be seen with any spectroscope, if we 
substitute a fluorescent for the ordinary eyepiece ; a cell containing 
a dilute solution of esculin must, however, be substituted for and 
placed in the position of the uranium glass plate of the eyepiece, 
uranium glass fluorescing most feebly in the light of the spectral region 
where the absorption-band under discussion is situated. It was, indeed, 
with the aid of his fluorescent eyepiece that Soret first discovered this 
band, though d'Arsonval asserts that it is impossible to see it in this 
way. Observations with the fluorescent eyepiece are, however, difficult 
and require experience. Still more difficult and unsatisfactory is the 
method, also suggested by Soret, and lately published as an original 
suggestion by d'Arsonval, of rendering this band visible by interposing a 
blue glass between the eye and the spectroscope. If the light be very 
intense the band is just perceptible to a person who is already 
acquainted with its position and characters through other methods of 
observation. 
In order to demonstrate Soret's band and the absorption-bands in 
the ultra-violet of derivatives of the blood-colouring matter, I projected 
the spectrum of sunlight or of the positive pole of the electric arc on to 
a fluorescent screen, similar to those which have since come into 
common use in observations made with the X or Eontgen rays, 
i.e. a screen made by coating a white surface, such as cardboard, with 
barium platinocyanide. 
In order to render absorption - bands of coloured liquids in the 
extreme violet and ultra-violet beautifully visible by this method, it is 
essential, however, to open the slit which intervenes between the 
source of light and the collimating lens very widely. In the highly 
luminous spectra thus obtained, though none of the spectral lines 
are visible, except perhaps H and K appearing blurred and indistinct, 
absorption-bands appear with remarkable distinctness and sharpness. 
The method is valuable, not only for purposes of demonstration, but 
for making preliminary observations prior to having recourse to 
photography. By its help I ascertained with correctness the position 
and characters of the extreme violet and ultra-violet absorption-bands 
of the acid compounds of haematin, of methtemoglobin, of haemato- 
porphyrin, and of turacin. In no case where this method yielded 
negative results, was the presence of a band afterwards demonstrated 
by photography. 
As few physiological laboratories possess a perfectly darkened optical 
