The Dispersion and .Ibsorption of Fuchsia 19 



the measurements within the absorption band, in which case a 

 tin disc, as shown in fig. 6, was used, having a slit cut in the edge 

 0.7 mm. wide and another, about a cm. from the edge, 1.5 mm. 

 wide. This disc was rotated very rapidly before the collimator 

 slit by means of a small motor, and thus the intensity of the light 

 incident upon the collimator slit was diminished by 99 856 or 

 99.682 per cent, according to which of the slits of the disc was 

 used. Since the edges of the slits of the disc are parallel, care 

 had to be taken to have the collimator slit at a definite distance 

 from the center of the disc. This was done by using the disc 

 close up against the collimator slit ; and then, by holding a sharp 

 edge on a line B or C scratched on the disc, the shadow of this 

 edge would fall on the center of the collimator slit, when the 

 center of the disc was at a distance OC or OB from the ray 

 entering the collimator. 



The use of this tin disc, which cut down the intensity of the 

 light so greatly, made it possible to match very accurately, al- 

 though the light had been reduced by the fuchsin to such an 

 enormous degree. Since the slope of the absorption curve was 

 very steep, it was necessary to have the collimator slit v? quite 

 narrow, not more than half a millimeter wide at most, in order 

 that the readings should be made with sufficiently homogeneous 

 light; otherwise the absorption as observed would have too low 

 a value. With the slit j half a millimeter wide, s' would have 

 been less than a five-thousandth part of a millimeter wide, when 

 the adjustment was made using no sector, and accurate settings 

 of the slit at this width would have been impossible. This fea- 

 ture of the photometer, together with the fact that a bare flame 

 or direct sunlight can be used instead of a flame or sunlight 

 behind ground or opal glass, which would cause a loss of 95 per 

 cent to 98 per cent, and the absence of Nicol prisms, which would 

 cause a further loss, makes it singularly well adapted for work 

 on strongly absorbing substances. 



The diminution in intensity caused by transmission through the 

 fuchsin film is due to two factors, reflection from its surfaces and 

 absorption within the film. To determine the absorption it is 

 necessary either to eliminate the reflection by measuring the dif- 



119 



