E. M. NELSON ON AN OPTICAL RULE. 209 



converging lens, is the power of the diverging lens. Its focal 

 length can then be determined in either inches or mm. by 

 inspection. Example : The power of a double concave, when 

 combined with a bi-convex, is 085, as measured on the P side 

 of the rale. The power of the bi-convex alone is 30 ; then 

 0*85, minus 30, equals minus 215. The focal length of the 

 double concave is therefore minus 4'65 inches or 118 mm., these 

 being the figures in a line with 2 15 on the rod. 



(4.) The rule is very useful as a ready reckoner. Example 

 (a) : A lens of 8| inches focus is combined with one of 94 mm. 

 focus — required the power and focus of the combination in 

 inches and mm. ; 8| inches is in a line with 1*175 P, and 94 

 mm. in a line with 2*7 P. The power of the combination is, 

 therefore, 1175, plus 2 7, equals 3875 ; this is in a line with 

 2 58 inches and 653 mm., the foci required. Example (6) : 

 I have a lens 178 mm. focus — what must be the focal length 

 of the lens in inches that added to it will yield a power of 

 5? In a line with 178 mm. is 1425 P; then 5 minus 

 1*425 equals 3"575, the power of the lens required ; this is in a 

 line with 2*8 inches, which is its focal length. 



Journ. Q. M. 0., Series II., No. 38- 15 



