106 



S. R. Williams — An Achromatoscope. 



and we can interpret at once the type of dispersion the medium 

 will show. For the sake of brevity these interference bands 

 may be called interference dispersion curves. The conditions 

 for anomalous dispersion are very interesting. If at any point 

 in the spectrum dfi/d\ = or < 0, we say that the substance 

 shows anomalous dispersion. This does not mean however 



Fig. 3. 



that points of inflection will be shown in the interference dis- 

 persion curves from the films of substances possessing anoma- 

 lous dispersion. In an anomalous region, if the index of 

 refraction decreases it will tend to broaden the bands while for 

 an increase of refraction it will tend to make the bands closer 



Fig. 4. 



together. Thus at G in fig. 5 the index has suddenly decreased 

 and we see the bands curve more sharply outward, while at F 

 where the index increases the bands have curved inward 

 toward the central band. 



Even in a non-dispersive substance the interference disper- 

 sion curves are uniformly receding from the central band and 

 this recession must be overcome in order to show points of 

 inflection in the curves we are dealing with. This much can 



