582 Transactions of the Society. 



The above table was computed by the formula ,„ „. , Tr , . 



1 5* 4686 X. W. A. 



The numerical coefficient was determined from the data found by 

 the extinction of the image of a minute point by reducing the 

 W. A. to '165. The size of the point was measured by a wide- 

 angled oil-immersion, and a W. A. of * 9, and was found to be 

 apparently ^^ inch. 



From this we have 6 • 6961 A • 165 = 50050. And } = 



6'6961>-'9 



•000003663. Employing this as a provisional correction, we find 

 the size of the point to be 42396 i n - Again, using this measure- 

 ment, we obtain a new numerical coefficient, viz. 5 ■ 6587, and finally 

 find the size of the point £0^75 in., and the coefficient 5*4686 as 

 stated above. In this calculation A is the reciprocal of the wave- 

 length, or the number of waves per inch, given at the head of" 

 each column in the table. 



