new Definition of Focal Length, fyc. 143 



ol the rule would of necessity abut against the photographic- 

 film, repeated measurements would be prejudicial to the latter. 

 However, if the negative from which the scales are made is 

 preserved, such scales can be easily and cheaply reproduced. 



A very accurate way of fixing the principal foci is to place 

 the combination at rest vertically on a plane mirror. Imme- 

 diately above it hold a sharply-cut point of stiff white paper 

 in a universal holder and adjust its position in the vertical 

 line until its image formed by light passed twice through 

 the lens-system coincides with itself. This adjustment may 

 be most accurately made by a short focal-length hand-lens. 



Type of a Convex System 



-« <iw2 krec{ion ef Light. 



4 3 2 »l , O -I -2 -3 



Concave Syste/ 



-2 



1 

 4 



1 

 3 



i 

 2 



CoA/CA VE 



1 







M 'iffff O R 



1 -2 



-J 



4 

 1 



3 

 1 



2 

 h- 





-1 



1 h 



-2 



1 



-3 



\ 



D- Coa/ve:* M/EfROR 



-2-10123*5 



-+- 1 1 h 1 1 + 



Note. — In all these diagrams light is supposed to come in from the right 

 hand ; the upper scale applies to the focus for the light before it 

 encounters the system, the under scale to the focus for the light 

 which has emerged from the system. The diagrams hold good if the 

 sheet is turned upside down. Any erecting system of lenses applied 

 to a given combiuatiou changes the character of the latter from 

 concave to convex and vice versa. 



