﻿Intelligence and Miscellaneous Articles. 483 



(Laupp, Tubingen), ' On the Application of Spectrum Analysis to 

 the Measurement and Comparison of the Intensity of Coloured 

 Light,' in which the spectra of solar light and of various terrestrial 

 sources of light are photometrically examined, and moreover mea- 

 surements of the luminous intensity of the bright lines of the spectra 

 of some chemical elements are communicated. 



The first problem in the quantitative determination of coloured 

 bodies by spectrum-analysis (a problem which has not hitherto been 

 put in chemical laboratories) consists in measuring the absorption of 

 light in any given region of the spectrum by a transparent body 

 brought before one half of the slit, the rest of the spectrum being 

 stopped off in the manner described. This is very simply effected 

 by narrowing the fore part of the slit by means of the correspond- 

 ing micrometer-screw until the luminous intensity is the same in 

 the upper as in the lower half of the spectrum investigated. If 

 the luminous intensity in the part investigated is very great, the free 

 half of the slit is first covered with a smoked glass of known darken- 

 ing-power, and then perfect equality of luminous intensity is pro- 

 duced by altering the width of the slit. 



Equality in luminous intensity in the two halves of the spectrum is 

 very quickly produced ; and thereupon the intensity of the light which 

 remains after traversing the coloured body brought before the slit is 

 directly found in a percentage. In some investigations the measure- 

 ment of the photometric values of the region investigated is also 

 desirable; this is effected by the method described in the above- 

 mentioned paper. 



As the eye can discriminate very small differences in the intensity 

 of monochromatic light, the method possesses all the guarantees of 

 an accurate objective measurement. It thus furnishes a simple means 

 of measuring the absorption of light in all regions of the spectrum ; 

 it is also suited to determine the contents of any given solution of 

 a colouring-matter. 



To the definition of the coefficient of the absorption of light, as 

 well as of the coefficient of extinction (E) of a solution, moreover 

 of the coefficient of absorption (A) of the dissolved active ab- 

 sorbing body, the following is to be added : the first and third of 

 these coefficients correspond to the definitions which have long been 

 used, while the coefficient of extinction is taken in the well-known 

 sense introduced by Bunsen (in his photochemical investigations). 



A definite relation between the coefficient of extinction E and the 

 concentrations of variously concentrated solutions of one and the same 



C 



coloured substance may be anticipated ; — is, in fact, nothing more 



E 



than the coefficient of absorption A of the dissolved substance, cor- 

 responding to the older definitions. 



If, then, for any given single place of the spectrum the coefficient 



Q 



of absorption of the coloured substance (that is, the value — ) and 



E 



hence the coefficient of extinction of a single solution of previously 



known concentration has been determined, any unknown concentra- 



