^1k 



Dr Brewster on a new series of 



In order to study this subject under a more general aspect, 

 I was desirous of examining the phenomena exhibited by 

 grooved surfaces of different refractive powers. It was obvi- 

 ously impossible to procure systems of lines upon transparent 

 bodies in which the grooves should have exactly the same dis- 

 tance and magnitude ; but I conceived it practicable to impress 

 upon different substances the very grooves which produced 

 the preceding phenomena, and I succeeded in impressing the 

 system of 1000 grooves upon tin, realgar, and isinglass. 



The following results were obtained with Tin, the colours 

 being those upon A B, Fig. 10. 



The following results were obtained with Isinglass, 

 colours were generally the same as in the steel. 



The first limit of pink and blue was at 75 45 



The blue of second order - - - 73 45 

 The second limit of pink and blue was at 54 30 



The 



