Dr Hartridge, Colourimeter Design 281 



tive to change in shade than it is to change in intensity. I have 

 found, further, that the point of greatest sensitiveness is obtained 

 when the fields are nearly neutral in colour. Such a condition is 

 obtained by the use of a suitable colour filter which absorbs in that 

 part of the spectrum which is occupied by the complementary 

 colour to that absorbed by the pigment. Suppose, for example, a 

 yellow pigment is to be estimated, then a blue solution of a dye is 

 placed in the path of the light from the source of such a thickness 

 and concentration that the comparison field seen in the instrument 

 is of a neutral grey colour. Permanent colour films between glass 

 should be used if much work is likely to be done with any given 

 pigment. Such a technique is very simple, and I find that in my 

 hands it increases the accuracy of the determinations by about 

 three times (when estimating sodium picrate), the method of mean 

 squares being used to calculate the average error of the experi- 

 mental determinations both with and without the complementary 

 filter. The probable error of the determinations was found to be 

 0"8 per cent., using home-made apparatus and the complementary 

 screen. It should be possible to halve this amount if the precau- 

 tions outlined above be taken and well-designed apparatus be used. 



Summary. 



(1) The comparison field seen on looking down the instrument 

 should cause the greatest contrast and at the same time should not 

 produce after images. 



(2) On both limbs of the instrument double troughs should be 

 used, so that the thickness of pigment to be measured may be 

 varied at will, while the absorption caused by other pigments 

 remains constant. 



(3) An artificial light source should be used, and the lighting 

 system be so designed that narrow beams are produced of just 

 sufficient width as to completely illuminate the comparison field. 

 The amount of reflected and scattered light may thus be reduced 

 to a minimum. 



(4) If experiment shows that the change in colour produced 

 by a given change in thickness or concentration of the pigment 

 can be increased by modifying the relative intensity of different 

 parts of the spectrum of the light source, then suitable colour filters 

 should be prepared for use during the determinations. It was 

 found in a test case that this modification alone increased the 

 accuracy by three times. 



(5) The general design of the instrument should conform to 

 microscopic practice, fixed troughs being supported by the stage 

 and the movable trough actuated by the rack and pinion course 



