finding the Focal Lengths of Mirrors and Lenses. 61 



corresponding values of u and v. This supersedes the use of 

 the formulae, or the recollection of the positions of the four 



Fig. 1. 









11 ■■ 1 



in 







■H» 







na ^KSiBBpTlO BI3 





















fixed points mentioned above, avoids the cumbrous calculation 

 of reciprocals, and at the same time exhibits a criterion of 

 the accuracy of the experiment. The student has simply to 

 (1) mark off along the axes of x and y with due regard to 

 sign the corresponding values of u and v as observed on the 



