THE COLOURED BLOOD-CORPUSCLES. 31 



between the light and the slit of the spectroscope place a 

 test-tube containing the blood or its solution. Focus the 

 long image of the gas-flame on the slit of the spectroscope. 

 The focal point can be readily ascertained by holding a sheet 

 of white paper behind the test-tube. 



(c.) Add 10 to 15 volumes of water, and note that only 

 the red part of the spectrum is visible. Make a sketch of 

 what you see, noting the dilution. 



(d.) Add more water until the green appears, and observe 

 that a single dark absorption band appears between the red 

 and green (Fig. 4, 1). Continue to dilute until this single 

 broad band is resolved into two by the transmission of 

 yellow-green light. Burn a bead of sodic chloride in the 

 gas-flame, to note distinctly the position of the D line, and 

 observe that of the two absorption bands the one nearest D, 

 conveniently designated by the letter a, is more sharply 

 defined and narrower ; while the other, conveniently desig- 

 nated by the letter (3, nearer the violet end, is broader and 

 fainter. At the violet end the spectrum is shortened by 

 absorption (Fig. 4, 2). 



(e.) Continue to dilute the solution, and note that the 

 band near the violet end is the first to disappear. 



Sketch the appearances seen with each dilution, and indicate 

 opposite each the degree of the latter. 



(f.) A very instructive method is to make a pretty strong 

 solution of blood, showing only one undivided band. Place 

 a little of this in a test-tube, and pour in water, so that the 

 water mixes but slightly with the upper strata of the blood. 

 Examine the solution spectroscopically, moving the tube so 

 as to examine first the deeper strata of fluid until the 

 surface is reached. At first a single band is seen ; but as 

 the solution is more dilute above, the two bands begin to 

 appear, and as the solution gets weaker above, the /3-band 

 disappears, until, finally, with a very weak solution, such 

 as is obtained in the upper strata only, the a-band alone is 

 visible. 



7. Hsematinometer. For accurate observation, instead of a 

 test-tube the blood is introduced into a vessel with parallel sides, 



