228 M. Preyer on the Analysis of Colouring -matters, 



sent when illuminated by homogeneous light. The stage of the 

 microscope is illuminated by the various elementary rays of the 

 spectrum. 



Yellow light is bright enough for making the most delicate 

 observations ; the others range as follows in the order of decrea- 

 sing intensity : — orange, red, green, blue, and indigo. Human 

 blood, or the blood of frogs placed successively in these various 

 rays, presents the following phenomena : — 



In red light the oxygenated globules appear of an intense red ; 

 in orange light and in the least refrangible parts of yellow they 

 are also red ; in the most refrangible yellow they are only red- 

 dish. When the field passes into green, the globules entirely lose 

 their red colour, and become completely black in the green near- 

 est the yellow. In the extreme green they are a little brighter. 

 In blue and violet the reddish tint reappears. 



It follows thence that the globules contain a substance which 

 absorbs green rays with the utmost energy; and as Hoppe-Sey- 

 ler's hsemoglobine has its greatest absorption in the green, it is at 

 least probable that this substance is hsemoglobine, thus furnish- 

 ing a fresh proof that this body really represents the colouring- 

 matter of the blood. 



From these facts a very delicate method may be deduced for 

 ascertaining the presence of blood. Three or four globules of 

 the colouring-matter are placed on the stage, and it is observed 

 whether they offer in the various homogeneous lights the phe- 

 nomena of absorption corresponding to the rays of absorption of 

 hsemoglobine — assuming in all cases that there is no other sub- 

 stance which behaves like hsemoglobine. Pigments which are 

 similar in colour and in optical properties can also be distin- 

 guished by this method. 



The substance is observed in crystals, or in amorphous masses, 

 which may be spread on cloth. The same point of the field is 

 fixed and the light varied ; the tints are noticed in which the 

 pigment loses its colour, deepens, becomes black, or retains its 

 greatest intensity. 



By this experiment a judgment can be formed of the appear- 

 ance of a spectrum seen through a solution of a body. Thus 

 the rays which produce the greatest obscuration will be either 

 wholly or partially wanting in the spectrum; conversely, the 

 spectrum of a colouring-matter being given, the presence of this 

 colouring-matter can be demonstrated by the appearance it pre- 

 sents in the microscope. 



Mitscherlich has described a new method of determining the 

 composition of organic compounds*. He has succeeded in find- 

 ing a method for determining directly all the elements of an or- 

 * PoggendorfF's Annalen, April 1867. 



