Gee and A damson, Dioptriemeters. 



In the case of lenses where great accuracy is not 

 required, in converting from inches into dioptrics a metre 

 is taken as 40 inches, thus we have : — 



Eocal Length. 

 Inches. 



Focal Lengtli. 

 Metres. 



Dioptrics. 

 Actual. 



Dioptries. 

 Approximate. 



40 



I 0-0254 39-37 



5 0-1270 7-87 8 



Generally, if the focal length be /"metres and its power 

 D in dioptries, then numerically D = \lf. The sign of D is 

 onsidered as positive for converging, and negative for 



o»a 



30 



20 



10 



Fig. I. — First Graphical Method for finding D. 



diverging lenses ; whilst in the convention of signs used 

 by most writers on geometrical optics the signs of the 

 focal lengths of the corresponding lenses are used in the 

 reverse way. If both conventions are adopted, the 

 algebraical relations between D and /is D = — i//. When/ 

 is known, the value of the reciprocal D can be found by 

 calculation, by using a table of reciprocals, or by a 

 graphical construction — such as shown in Figs, i and 2 — 

 where the value of D may be read directly on any scale of 

 equal parts. 



