394 
Journal of Agricultural Research 
Vol. XXVI, No. g, 
For the experiments in this paper each cell had an inside measurement 
of 2 cm. The 2 cm. refers to the thickness of the solution through which 
the light passes and not to the diameter of the cell. One cell is filled with 
the carotin solution and the other with the solvent. The cells are then 
placed in the holder on the instrument. In measuring transmittancies, 
as in this investigation, the cell containing the carotin solution is placed 
in one beam of light, and a similar cell, containing the solvent, is placed 
in the other beam. These cells are interchanged and the nicol is rotated 
in each case until an intensity match is obtained. 12 The transmittancy 
Fig. 4.—Lovibond slide readings plotted to show the results obtained from different concentrations 
of carotin in ether. 
is then obtained as the product of the cotangent of the angle in one case 
by the tangent of the angle in the other. 
With an ethereal solution of carotin, using the mercury line 435.8, 
angles were read as follows: 0 V 64.18; 0 2 , 16.43. The cotangent of 0 t 
times the tangent of 0 2 gives the value 0.143 f° r the transmittancy of the 
solution. At the left-hand side of Figure 3, the value 0.143 t° r the trans¬ 
mittancy is found. This point on the graph represents 2.12 mgm. of caro¬ 
tin per liter. The transmittancy/and consequently the concentration, of 
any solution of carotin of unknown concentration may be obtained in 
this way. 
1S For each position of the cells there are four settings of the nicol for which an intensity match can be 
obtained; that is, one in each quadrant of the circle. The instrument was adjusted so that the readings 
could all be taken in the first quadrant; usually io settings of each angle of match were made. In using 
the homogeneous light source, such as was used in these experiments, the ocular and the collimator slits 
were opened to 0.5 mm. and the field brightness thus obtained was ample in all cases to make good settings 
of the angle of match. 
