The calculations are effected as follows :— 



Problem /.—To find the necessary focal length of the lenticula 



and RN = a = one-half the 



'. get 



ce the distance of the object P = R P = y(p- 

 id the distance of the image Q = R Q = s'{(\' 



f ;/(p;+a^) V(q^+a=) 



whence the focal distance necessary (f) 



^- V((r+a^)-./(p^+a^T 

 which is the foi-mula required. 



Problem //.—To find the prismatic angle necessary. Sine 

 actual pencil of light that enters the pupil of the eye is very £ 

 and the ray P R M is the axis thereof, we may consider the 

 P R Q as the angle of deviation. 



NowanglePRQ = NRQ - NRP 



.-. tanPRQ = tixn(NRQ - K R P) 



that !>, the angle PR<.) tiir' ^ '_^;^^ 

 ind cm l.e found fiom tibl s 1. M,,un<<l i h. pn-nntu m 



of i< 1 1 action) 



Tlie pusiii itic nude dots not iii( in t lie angle BAG foimod 



jdus on( 01 moie lenses, as shown in faguie 3, wheie a bi com 



