﻿426 Mr. A. Ferguson on Photographic 



curvature at some point (x, y) in the neighbourhood of the 

 important terms, and applying the relation 



ik+iy^-^- 



A numerical example will make the matter clear : — 

 Using the above data, take 



x=Vm 7-166 ^ = 981 S 



y=-257o Magnification;^, 



^ = •11 Y 4 P = 1 — 



c.c. 



Then 



1 2b 2x-1174x7-166 



Ei 



_ (l+46 2 c 



r»)l 



~ (1-1216)1 x 



= 3-175, 



•1459 



1 



2b 





2x-1174x 

 ~~ (1*1216)* x 



7-166 



R 2 



~ (l + 4£ 2 . 



0* 



•4459 



= 3-561, 



1 1 - 7Q , 



. , -2575 x -4459 

 True value or ?/ = ■ ,. , .,,. ='01bU, 



7i-y=:-5186-*0160=-5026; 



T 981 x -5026 _„ g , _, 



T=- — ; — =73*2 dyne-cm. \ 



b'7op 



A similar calculation using the coordinates # = l"51o, 

 #=•2705, gives 



A-y=-5018 



and therefore T = 73'4 dyne-cm." 1 at a temperature of 11° 0. 

 Turning now to the biquadratic equation y = b.c 2 + cj a , in 

 computing b and c the same coordinates are used as in 

 Table I. Some additional products are required and these 

 are given in Table II. 



