THE PHOTOGRAPHIC SPECTRUM. 227 



room provided with a heliostat for projecting a beam of sunlight into it, 

 the following simple arrangement, 1 which requires merely an electric 

 arc lamp and an ordinary laboratory spectroscope of the Bunsen type, 

 may be adopted. 2 The telescope of the spectroscope is removed, and 

 a beam from the + pole of the arc is allowed to fall on the slit of the 

 collimator. The spectrum is focussed on a fluorescent screen, then the 

 slit is opened very widely. If the spectrum be a continuous one 

 (which is the case if it be that of the positive pole of the electric 

 arc), the coloured solution is then interposed in the path of the beam 

 falling on the slit. 



The position and limits of Soret's land. Defibrinated arterialised 

 blood, diluted with from 400 to 600 volumes of distilled water, or still 

 better with a similar amount of 0*1 per cent, solution of sodium hydrate, 



G h HK L M N 



FIG. 33. The photographic spectrum of haemoglobin and oxyhsemoglobin. 



furnishes solutions (containing about 1 part of oxyhsemoglobin in 3000 

 and 1 part in 5000 respectively) of a concentration suited for 

 photographic investigations of the spectrum. With solutions of this 

 strength (a stratum 1 cm. thick being placed in the path of the beam 

 falling upon the slit of the collimator) Soret's band can be studied 

 to perfection, though it can be well seen with solutions much more 

 concentrated and much more dilute. The appearance and position of 

 Soret's band in the spectrum of oxyhaemoglobin are shown in Fig. 33 

 along with that of reduced haemoglobin. 



Within fairly wide limits of concentration (the stratum examined 

 being invariably 1 cm. wide), the limits and characters of Soret's band 



1 I employed this simple arrangement in demonstrating these bands in the violet and 

 ultra-violet to members of the Internat. Physiological Congress, Bern Meeting, September 

 1895. 



'-' Direct vision spectroscopes cannot be used, the absorption of the ultra-violet rays being 

 very great in these instruments. 



