CH. XXIX.] THE PHOTOGRAPHIC SPECTRUM 469 



is not visible to the eye, but can be rendered visible by allowing the 

 spectrum to fall on a fluorescent screen, or on a sensitive photographic 

 plate. In order to show absorption bands in this part of the spectrum 

 very dilute solutions of the pigment must be used. 



Oxyhaemoglobin shows a band (Soret's band) between the lines G 

 and H. In haemoglobin, carbonic oxide haemoglobin, and nitric oxide 



FIG. 335. The photographic spectrum of oxyluerooglobin and methsemoglobin. (Gamgee.) 



haemoglobin, this band is rather nearer G-. Methaemoglobin and 

 haematoporphyrin show similar bands. 



We owe most of our knowledge of the " photographic spectrum " 

 to the late Prof. Gamgee, through whose kindness I am enabled to 

 present reproductions of two of his numerous photographs (figs. 

 334 and 335). 



Methsemoglobin. This may be produced artificially in various 

 ways, as by adding potassium ferricyanide or amyl nitrite to blood, 

 and as it also may occur in certain diseased conditions in the urine, 

 it is of considerable practical importance. It can be crystallised, and 

 is found to contain the same amount of oxygen as oxyhaemoglobin, 

 only combined in a different way. The oxygen is not removable by 

 the air-pump, nor by a stream of neutral gas such as hydrogen. It 

 can, however, by reducing agents like ammonium sulphide, be made to 

 yield haemoglobin. Methaemoglobin is of a brownish red colour, and 

 gives a characteristic absorption band in the red between the C and 

 D lines (spectrum 7 in coloured plate). In dilute solutions other 

 bands can be seen. 



Potassium ferricyanide is the most convenient reagent for making methaemo- 

 globin. It is, however, necessary to remind the reader that it produces another 

 effect as well, namely, it causes an evolution of gas, if the blood has been previ- 

 ously laked. This gas is oxygen ; in fact, all the oxygen combined as oxyhanno- 



