60 Mr Filon, On the variation with the wave-length 



, D Wcc + wP 

 where F = — - + w , 



W = weight and its hanger, 

 a; = distance from the fulcrum of knife-edge supporting 



weight, 

 w — weight of lever, 



£= distance of c. of G. of lever from fulcrum, 

 w' = weight of knife-edge block. 



For the instrument used 



w' = 97 gms. 

 w = 977 gms. 

 £ = 22'75 cms. 



Hence 



X=^ (22-52 + Was), 



W being in kilogrammes weight. 



The block of glass was a piece cut from one of the beams 

 ordered for the earlier experiments. 



a = T231 cms. 



b = 2'91 cms. 



c = 3*71 cms. 

 the density being about 2 # 50. 



The results of the experiments are shown in the tables below: 



Table I. 

 July 8th. as = 40. 



