6 1 MICRO-SPECTROSCOPE AND POLARISCOPE. 



and thicker layer may be seen. Note especially that the two charac- 

 teristic bands unite and form one wide band as the stratum of liquid 

 thickens. Compare with the following : 



Add to the vial of diluted blood a drop or two of ammonium sul- 

 phide, such as is used for a reducing agent in chemical laboratories. 

 Shake the bottle gently and then allow it to stand for ten or fifteen 

 minutes. Examine it and the two bands will have been replaced by a 

 single less clearly defined band in about the same position. The blood 

 will also appear somewhat purple. Shake the vial vigorously and the 

 color will change to the bright red of fresh blood. Examine it again 

 with the spectroscope and the two bands will be visible. After five or 

 ten minutes another examination will show but a single band. 

 Incline the bottle so that a very thin stratum may be exam- 

 ined. Note that the stratum of liquid must be considerably thicker to 

 show the absorption band than was necessary to show the two bands 

 in the first experiment. Furthermore, while the single band may be 

 made quite black on thickening the stratum, in will not separate into 

 two bands with a thinner stratum. In this experiment it is very in- 

 structive to have a second vial of fresh, diluted blood, say that from 

 the watch-glass, before the opening to the comparison prism. The 

 two-banded spectrum will then be in position to be compared with the 

 spectrum of the blood treated with the ammonium sulphide. 



The two banded spectrum is of oxy-haemoglobin or arterial blood, 

 the single banded spectrum is of haemoglobin (sometimes called re- 

 duced haemoglobin) or venous blood, that is the respiratory oxygen is 

 present in the two banded spectrum but absent from the single banded 

 spectrum. When the bottle was shaken the haemoglobin took up 

 oxygen from the air and became oxy-haemoglobin, as occurs in the 

 lungs, but soon the ammonium sulphide took away the respiratory 

 oxygen thus reducing the oxy-haemoglobin to haemoglobin. This 

 may be repeated many times. For further spectroscopic study of blood, 

 see Part II. 



§ 147. Colored Bodies not giving Distinctly Banded Absorp- 

 tion Spectra. — Some quite brilliantly colored objects, like the skin of 

 a red apple, do not give a banded spectrum. Take the skin of a red 

 apple, mount it on a slide, put on a cover-glass and add a drop of water 

 at the edge of the cover. Put the preparation under the microscope 

 and observe the spectrum. Although no bands will appear, in some 

 cases at least, yet the ends of the spectrum will be restricted and vari- 

 ous regions of the spectrum will not be so bright as the comparison 

 spectrum. Here the red color arises from the mixture of the unab- 

 sorbed wave lengths, as occurs with other colored objects. In this 

 case, however, not all the light of a given wave length is absorbed, 





