280 Dr Hartridge, Colourimeter Design 



may be the more important : for example, a pigment absorbing to 

 an equal extent in two complementary parts of the spectrum will 

 cause the light to suffer no change in colour at all, while the in- 

 tensity is altered. (3) The changes in the case of any one pigment 

 can be increased by increasing the intensity of that part of the 

 spectrum which is suffering change or by decreasing that of parts 

 which do not show alteration. Of the two methods the latter is 

 the easier to carry out and the more efficient. If colour filters are 

 used they must be carefully adjusted according to the position in 

 the spectrum of the absorption band of the pigment to be estimated. 

 If a spectral illuminator is used the apparatus virtually becomes 

 a spectrophotometer, and this elaboration is hardly necessary for 

 ordinary work. The possibility should not be overlooked of the 

 existence of alternative colour reactions to those at present in use 

 in which pigments having less steep absorption bands are used and 

 which therefore permit greater accuracy in their colourimetric 

 estimation. 



The factors which influence the acuteness of perception of the 

 eye remain for consideration. Firstly, it is clear since the accuracy 

 of the determination depends on the correctness of the match ob- 

 tained, that the eye should not be suffering from fatigue. The 

 reading of small print and the exposure of the eyes to excessive 

 light should, therefore, be avoided for a reasonable time before the 

 determinations. The absence of refractional errors, eye strain, want 

 of eye-muscle balance and the possession of good general health are 

 all factors of importance. In my own case the period after tea is the 

 best, provided that the morning's work has not been arduous. The 

 presence of after images is most harmful for accurate estimations ; 

 the best method of eliminating them is, I find, to look for a fcAV 

 moments at a uniformly lit grey surface. All the above points may 

 seem obvious ; it is however my experience to find that they are 

 sometimes overlooked. The apparatus itself is best placed in a dark 

 room, or at all events where the full light of a window cannot fall 

 on the eye of the observer. In the latter case the eyepiece cup 

 may be made deep with advantage, so as to protect the periphery 

 of the retina from stimulation and thus bring about an increase in 

 the diameter of the pupil. 



With regard to the use of colour filters, experiment shows that 

 the theoretical conclusions arrived at above are amply justified, 

 namely, that the accuracy of the determinations is increased if 

 either the rays absorbed by the pigment are increased in intensity, 

 or those not absorbed are decreased or removed altogether. The 

 removal by means of colour filters is however usually attended by so 

 great a diminution in the intensity of the light that a powerful 

 source such as an arc lamp becomes necessary. It is a fortunate 

 circumstance, therefore, that the retina should be even more sensi- 



