Dr Harfridge, Colourimeter Design 275 



a focussed image of the diaphragm of the light source, which is 

 magnified in the ratio of the focal lengths of the lenses; since the 

 ratio is 3 to 1 this image has a diameter of 12 mm. 



The beams that emerge through the lenses Tl and T'2 do not 

 therefore anywhere exceed 12 mm. and the light does not spread, 

 for this reason, to the sides of the troughs during its passage 

 and therefore stray light is reduced to a minimum. The beam 

 from the lens T2 passes vertically upwards through a hole in 

 the stage to the standard trough which rests upon it. Having 

 passed through both the layer of solvent and also that of the 

 solution of pigment, the beam enters prism B', and is totally inter- 

 nally reflected at its inclined surface on to the silvered strips of the 

 comparison field. The beam that has passed through Tl is deflected 

 by internal reflection at the right angled prism C which is cemented 

 to it, and falls on the silvered sui'face between the two halves of the 

 prism D, so that the beam is directed vertically through a second 

 hole in the stage on to the lower fixed cup of the adjustable trough, 

 which is filled with solvent. It then passes through the movable 

 cup which contains the pigment, and enters the prism A to fall on 

 the silvered strips of the comparison field. The passage of this 

 beam through the intervals between the strips, and the reflection 

 of the beam from the other limb of the instrument at the strips 

 themselves, has already been described. It will be noted that the 

 reflection of the one beam by internal reflection within the prism 

 C, and by ordinary reflection within the prism D, causes this beam 

 to compensate for the internal reflection and reflection at a silvered 

 surface which occurs within prism B in the case of the other beam. 

 As it has been found that silvered surfaces vary in the intensity 

 of rays of different wave-length which they reflect, it is advisable 

 that both mirror D and prism B be silvered with the same solution 

 at the same time. 



The lengths of the paths of the beams through the instrument 

 are found to be in the case of the left-hand beam an actual dis- 

 tance of 19'5 cms., that is an effective distance of 18 cms. since 

 2"2 cms. of glass is passed through ; in the case of the right-hand 

 beam the total and the equivalent lengths are the same as those 

 on the left. 



The comparison field therefore is illuminated by two super- 

 posed images of the diaphragm of the light source, one of which 

 has passed through the standard trough and the other through the 

 adjustable trough. When the instrument is in correct adjustment 

 these two images exactly coincide, so that if there should be any 

 slight inequality between the intensity of illumination of different 

 parts of the light source both images will be similarly affected, 

 and therefore the match between their different parts will remain 

 unchanged. Such a condition is not secured in the usual forms of 



