60 Dr Brewster on a new series of' 



No. of grooves Orders and portions of orders of colours developed 



in an inch. from 90° up to 0* of incidence. 



5000 on wax One complete order, together with bluish white 

 of the second order. Colours more dilute 

 than in No. 5. 



10000 One complete order, together with blue and 



fainter blue of the second order. 



It is obvious from the preceding table that the diversity of 

 effect produced by different specimens does not depend upon 

 the quantity w -|- w, but upon ?i. The more that the original 

 surface is ploughed away by the cutting diamond, the more 

 briUiant were the tints, and the more numerous the orders of 

 colours. 



T was now desirous of seeing what effect would be produced 

 when the original surface was almost wholly removed; and 

 Mr Barton was so obliging as to execute for me a specimen 

 containing 2000 grooves in an inch, in which this was nearly 

 effected. His diamond point, however, having unfortunately 

 broken before he had executed any considerable space, I was 

 unable to make all the experiments with it which 1 could have 

 wished. 



This specimen produced four complete orders of colours, all 

 of which were developed at much greater angles of incidence 

 than those in the preceding Tables. 



White - - 90 00 Green. 



Straw yellow. Yellowish green. 



Faint red. Yellow. 



Fink. Orange. 



First limit of pink and Scarlet, 



blue - 80 00 Purple. 



Blue. Third limit of pink and 



Green. blue 48 00 



Yellow. Blue. 



Red. Brilliant green. 



Pink. Yellowish green. 



Second limit of pink and Yellow. 



blue - 69 40 Jleddish - - 10 00 

 Blue. 



