METHODS OF SPECTROPHOTOMETR Y. 219 



We shall suppose this result to have been attained, and next direct our atten- 

 tion to the relative illumination of the two spectral areas under examination. 

 It will be at once seen that the interposition of the absorption-cell has brought 

 about a great difference in this respect. The upper spectrum is seen to be 

 much less bright than the lower, the difference depending upon the amount 

 of colouring matter in solution. Unless the concentration be excessive, we can 

 restore the equality of illumination of the two superposed spectral areas by 

 narrowing the lower slit. This is done with great care until we are convinced 

 that the luminous intensity is the same in both, or that we have secured the 

 greatest attainable equality (see, below, the discussion of the objections to 

 Yierordt's method). We have then merely to read the division on the 

 divided circle of the micrometer screw of the lower half of the slit. Supposing 

 we find that the width of the lower slit is represented by division 20, whilst 

 the width of the upper slit remains at 100, then the former number represents 

 the percentage of unabsorbed light. We have seen that the extinction- 

 coefficient e can be determined by the formula 



= -log /', where /' 



represents the unabsorbed light. By a table of logarithms, or more quickly by 

 special tables, we find that in our case 



=- log T ^ = log 100 -log 20 

 6 = 0-69897 



As has been shown, having determined e, we may, if we know the precise 

 proportion of colouring matter contained in the coloured solution, calculate the 

 value of A ; or supposing that the substance is one of which the value of A 

 has been determined, and that we are unacquainted with its concentration, we 

 can ascertain the latter by the formula c = A. 



Although Vierordt's method of determining the extinction-coefficient 

 possesses historical interest, and its study is the natural introduction to that 

 of the more perfect methods which have been suggested by it, it is open to 

 serious objections, to the principal of which reference may here be made. 



However wide one slit may be, and however much the other may be 

 narrowed, it is, in the case of solutions of high colorific intensity, most difficult, 

 or impossible, to obtain by these means alone equality in the illumination 

 of the spectra ; and accordingly Vierordt frequently had recourse to the use of 

 smoke-tinted glass plates (Rauchglciser) of previously determined absorptive 

 power, these being interposed in the path of the light which had not traversed 

 the coloured solution. There are unquestionably theoretical and practical 

 objections to this mode of proceeding. The principal objection to Yierordt's 

 method is, however, a fundamental one, namely, that no absolute comparison 

 is possible between spectra obtained with slits varying considerably in width. 

 The more the slit of a spectroscope is widened, not only does the amount of 

 light admitted increase and the spectrum become brighter, but the more and 

 more impure does it become, i.e. the greater the admixture of light of different 

 wave lengths in any region of the spectrum. But the accurate determination 

 of the coefficient e is only possible with monochromatic light. It has been 

 sought to diminish the error due to the cause just referred to by substituting 

 for the original double slit of Yierordt one of which both edges move symmet- 

 rically, so that the centre of the slit remains in a constant position. Although, 

 doubtless, the error is reduced in this way, it is not entirely corrected. 



Although Yierordt's method of determining the value of the co-efficient e 

 will probably fall in future into disuse, his great merit of having been the 

 first to work out a method of spectrophotometry admitting of considerable 

 accuracy, and of having discovered and established its applicability to the 

 quantitative analysis of colouring matters, will always endure. 



Hiifner's method. This method, which has been made more and more 



