in Theory and Practice. 373 



Hence, if the camera-box is ouce put in focus, it stays so, 

 wherever it is moved along AC. 



III. Suppose the two beams AB and AC make an angle 

 irl'l-e with each other (fig. VIII.). 



As before, 



And 



if 6 is small, 



E. + H cos v—p cos^ V 



pcos {v—6)=^qob9 ; 



p cos v= E,— p sin V tan 6 ; 



pR 

 " pR + Rp sin V tan ^' 



= p(l— sin I' tan ^). 



If camera-box is put in focus when j/ = 0, it will be out of 

 focus at any point by an amount, 



y=p(l— sin j/tan^)— p= — psin vtan^. 

 But -^^ sin V 



.*. 7/=- — X sin 6= — X tan 0, 



the equation of a right line making an angle 6 with axis of a; 

 (fig. IX.). 



IV. Suppose the grating turned on its axis so that its radius 

 of curvature makes a constant angle a. with the arm BC 

 (fig. X.). 



BD=p, 



BC = a. 



Since /JL is kept equal to «, 



_ Rpcos^a 



R(cos a + cos v) — p cos''^!'* 



But a cos (« + v) = R 



a cos (a. + v) cos^a 

 a cos (a + v) (cos a + cos v) — p cos^v 



Put a=p (1 + B), and suppose both « and S to be small. Then 



r=p (l + « sin v—S cos v). 



Let the camera-box be placed in focus when v = ; the 



