RED GLOBULES OF THE BLOOD. 249 



spectrum gradually diminishes, and ceases altogether about the termina- 

 tion of the blue, midway between F and G. 



If the solution or mixture be much concentrated, or be viewed in a 

 very deep layer, it is too opaque for spectroscopic examination, and 

 may shut off all the light of the spectrum except a little of the red and 

 orange; if it be too dilute, it will fail to exhibit the distinguishing 

 characters of hemoglobine. A solution of a certain grade of strength, 

 which allows an abundance of light to pass through, and is yet sufficient 

 to cause its marked absorption at particular points of the spectrum, is 

 to be used for examination. With pure hemoglobine, according to 

 Preyer, a solution of about 1.5 parts per thousand gives the most marked 

 results. With fresh blood, if one volume of the defibrinated blood be 

 diluted with one hundred volumes of water, and the mixture viewed in 

 a layer of one centimetre, all the characteristic traits of the spectrum 

 will be distinctly shown. 



The spectroscopic characters above described form a very delicate 

 test for the coloring matter of blood. According to Preyer, with a 

 solution of pure hemoglobine in water, of 4 parts per ten thousand, the 

 absorption bands may still be seen, though the second one is very faint. 

 Fresh dog's blood, if diluted with 1000 parts of water and viewed in a 

 layer of 3 centimetres' thickness, will show a spectrum in which both 

 absorption bands are distinctly perceptible though not very strong. If 

 diluted with 10,000 parts of water and viewed in a layer of 4.5 centi- 

 metres, the first band is still visible, though very faint ; the second is 

 entirely imperceptible. 



These characters are also of value in showing that hemoglobine, as 

 extracted in the crystalline form, is identical with the normal coloring 

 matter of the fresh globules. A solution of crystallized hemoglobine 

 gives the same spectrum with solutions of fresh blood or with the dried 

 globules. EA^en the blood while still circulating in the vessels may be 

 made to exhibit the same appearances. If a spectroscope eye-piece with 

 two prisms be attached to the body of a microscope in such a way that 

 two spectra may be seen in the field, one above another, one of them 

 formed by the light coming through the body of the instrument, the 

 other by that coming through a lateral opening in the eye-piece ; and if 

 the mesentery of a living frog be placed before the objective of the micro- 

 scope, while a solution of human blood is placed at the lateral opening, 

 it will be seen that the absorption bands in the two spectra are the same, 

 and exactly correspond with each other. 



In all the above cases the blood which yields the characteristic spec- 

 trum already described is in the aerated condition. Even if venous blood 

 be taken for examination, the process of extracting it and placing it in 

 a suitable vessel for examination brings it in contact with the atmosphere 

 and thus restores it to the condition of arterial blood. In the mesentery 

 of the living frog, when extracted from the abdomen and spread out 

 under the microscope, the free access of air to the peritoneal surface 

 constantly supplies the circulating blood with oxygen ; and accordingly 

 17 



