Dr Hartridge, Colourimeter Design 273 



described above, are similar in shape to those used in the spectro- 

 photometer. They are shown in the diagram of the apparatus. It 

 will be observed that the interface of the prisms is silvered, the 

 metallic film being removed by means of a simple ruling machine, so 

 that narrow strips of the silver alternate with strips from which the 

 whole of the silver has been removed. Examination of the diagram 

 will show that by this arrangement the field seen on looking down the 

 eyepiece is formed of alternating narrow beams which have either 

 been transmitted from one limb of the instrument through the 

 spaces between the silver strips, or reflected from the other limb 

 by the silver strips themselves. The lengths of the prisms A and 

 JB should be such that the two entering beams have passed through 

 equal lengths of glass. 



The troughs are adjustable on both limbs of the instrument, 

 in colourimeters of usual design. This arrangement has the dis- 

 advantage that if there should be any backlash in the micrometer 

 mechanism which is used for adjusting the position of the movable 

 troughs, or error in the setting of the scale, these will affect both 

 the thickness of the pigment solution to be estimated, and also 

 that of the standard. Such errors can be eliminated so far as the 

 standard is concerned by the use of a special cell, the distances 

 between the sides of which are determined by accurately ground 

 distance pieces, which may be made of either glass or metal. 

 Rustless steel would appear to be a suitable metal because it resists 

 the corrosive action of ordinary solvents. 



I have shown that in the case of the spectrophotometer there 

 are important reasons for the use of troughs with double compart- 

 ments on both limbs of the instrument. In both ti'oughs the com- 

 partment near the light source should contain the solvent only, 

 the other being filled with the solution of the pigment. Double 

 troughs should be used with the colourimeter for similar reasons, 

 namely, (a) in order that absorption by the solvent may be com- 

 pensated, since the thickness is the same on both sides of the 

 instrument ; (b) that pigments accompanying the one under esti- 

 mation may be compensated for; (c) that specific surface reflection 

 at the sides of the troughs which contain pigment may be similar 

 on both limbs of the instrument. With regard to the type of 

 trough that should be employed I have previously considered the 

 advantages of the double wedge trough in conjunction with the 

 spectrophotometer. In the case of the colourimeter the plunger 

 type usually employed has the advantage of not requiring calibra- 

 tion with a micrometer microscope as wedge troughs do. The 

 method of employing double compartment plunger troughs and 

 standard troughs is shown in the diagram. In some colourimeters 

 the troughs are bell mouthed, and are manufactured from black 

 glass. These points are to be recommended. It should be noted, 



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