226 HEMOGLOBIN 



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 

 oxyhsemoglobin. 



Methods of demonstrating the land 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 sesculin 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 hasmatin, of methaemoglobin, of hsemato- 

 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 



