272 DR. ROYSTON-PIGOTT. 



= -0624975 

 /, = -0617284 



-0007691 

 Error, -roif^ths, or '00077. 



/ = -0624975 

 Virth = -0625000 



Difference = -0000025 

 between -yVtli the actual focal length and the nominal -^^h, 

 which amounts to an almost inappreciable quantity of 

 ffths of an inch. No English maker could pos- 



1 ooooooo' 



sibly make a -^6^^ object-glass which magnifies 1600 times 

 with a C eye-piece at 10 inches more accurately than this 

 standard. 



Example 4. — " One-eighth" nominal. 



r_ 10 10 



= 0-12193263 

 ±th = 0- 12500000 



Difference = 0-00306737 

 or -p^-Q-ths of an inch. 



But, if we take a " two-inch " or " three-inch/' then the 

 focal length is considerably shorter than the nominal, as the 

 conjugate focus is more widely separated from the solar for 

 parallel rays. 



Example 5. — " Two-inch :" magnifying power 50 at 10 

 inches, and 5 without the C eye-piece. 

 f = ^Q ^ ^ 



_ 12 

 ~ 9-2 



= 1-42857. 

 "Which is less than H inch. 



Resume. 

 N ominal objective. Solar focal length for parallel rays. 



2-inch . . 1^ inch, less ^^ths. 



1 I 7 tbq 



2 J» 10 o''**°" 

 rthS. 



1 0" 



3 fVio 



10 '"'^* 



