484 PEACTICAL PHYSIOLOGY. 



some portion will exist, when the amount of light passing from the two 

 sources is equal. Let the angle when this happens be called 0. Now, 

 suppose a cell of 1 cm. internal width, containing an absorbing 

 substance, be placed between L and M, some new position of the 

 pointer which may be called O l will now permit equal passage of light. 

 A certain amount of light (k) will be lost by absorption of the substance, 

 and this may be ascertained from the equation 



The light absorbed in different portions of the spectrum by a solution 

 of haemoglobin may thus be estimated. 



(1) To estimate the concentration of an absorbing substance the 

 method suggested by Lea may be adopted. 



A standard solution of the substance is examined in a cell of known 

 thickness, and the position of the pointer determined when the intensity 

 of the two images is identical. The unknown strength is then taken 

 and placed in a cell, the thickness of which can be varied. This is 

 substituted for the standard. The thickness of the cell is adjusted till 

 the position of equal intensities is the same as when the standard was 

 under examination. 



If c be the quantity of absorbing material in a unit of volume, and 

 M the thickness of the cell in the case of the standard, and c 1 and m l 

 in the case of the unknown strength, then 



ci= 



ra 1 



Thus the construction of the unknown can be estimated. 



(2) If a cell of variable thickness be not available a definite quantity 

 of the unknown solution may be placed in the cell of known thickness. 

 This is diluted (if stronger than the standard) with a known quantity of 

 the diluent till the position of the pointer is the same as when the 

 standard was examined. This affords a more delicate means of ascer- 

 taining exact quality of strength than the naked eye. From the 

 amount of diluent added the strength of the original absorbent can be 

 calculated. 



(3) One other method of estimating concentration is available. 

 Place a vessel of 1 cm. thickness containing the standard between one 

 Nicol and the source of light and ascertain B v the angle where equality of 

 brightness obtains. Remove the standard and substitute the unknown 

 strength of absorbing substance in a similar cell and ascertain the angle 

 (0 2 ) where equal brightness is observed. Let B be the angle of 

 equality when no absorbing medium is being examined. If C be the 



