334 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



apparatus carried round together, till the spectrum 

 is brought on to the screen. Now take a vessel, the 

 front and back of which are formed by two plates of 

 plane glass fixed parallel to one another, and only a 

 few millimetres apart. Into this pour a dilute solu- 

 tion of blood, and hold it in front of the slit. The 

 layer of blood will absorb certain rays, and the 

 spectrum of haemoglobin will be thrown on the screen. 

 The solution can easily be diluted to the strength 

 that gives the two bands sharply defined. Reducing 

 agents can be added to the blood in the vessel, and 

 after sufficient time has elapsed the band of reduced 

 haemoglobin will be obtained. 



EFFECTS OF THE SPECTRUM. 



Various observations have shown that the spec- 

 trum possesses (besides illuminating) heating and 

 chemical properties. These different properties are 

 not limited to definite regions of the spectrum. No 

 matter how great the dispersion, the illuminating 

 part of the spectrum cannot be separated entirely 

 from the heating portion, nor either of these two 

 absolutely from the chemically active part. But one 

 property is more intense in one part, and another 

 property in another part. 



The illuminating effects of the spectrum were 

 shown to attain their maximum in the yellow portion, 

 and to shade off at each side, but to be least in the 

 violet end. This is sufficiently indicated to the eye 

 by the sombre hue of the violet end of the spectrum, 

 and the brilliancy of the orange and yellow part, a 

 brilliancy even beyond that of the red, and specially 

 of the extreme red. 



The heating effects of the spectrum were first 

 shown by Herschell, in 1800, to be specially marked 

 in the red end. There are various ways of proving 

 this fact. A galvanometer attached to a thermopile 



