372 JULIUS KI1KINBERG OX STEREOSCOPIC EFFECT AND A 



directly with the size of the free aperture of the lens, and that 

 is the whole reason why slopping down a lens gives greater depth 

 perception. This, then, is the first point which we note about the 

 diffusion discs- one of common knowledge, universally recognised. 



Tl ; thing to note is not so generally recognised. Whilst 



i„ tl of iii«' point, which is in true focus at the image- 



plane, the rays which have traversed every part of the objective 

 are united, this does not hold good in the case of the diffusion 

 disc. The differenl parts of those discs are formed from light 

 which has traversed the different parts of the objective. This 

 may be seen from Fig. 1, the dark shaded part showing the 

 course of the light passing through a certain part of the objective 

 on the right hand side. Looking at the view plane, it will be 

 erved thai the corresponding portion of the diffusion disc, 

 which, coining from the nearer point, arrives there before coming 

 to its true focus, is on the right hand of the centre of the disc ; 

 whilst from the further point — which has already passed its 

 true focus when it arrives at the image plane — the shaded 

 portion of the diffusion disc is on the left of the disc centre. 

 When we want to name the position of any symmetrically shaped 

 disc we naturally specify its central point, so we see that, if 

 instead of utilising the whole objective we make use of a lateral 

 pari only, such as the shaded portion, the diffusion discs on the 

 image plane get shifted towards the same side as the portion of 

 the objective used when the object points which they represent 

 are on a plane nearer to the one in true focus, and towards the 

 opposite side when the object points are on a plane further away 

 than the one in true focus. We further see that the amount 

 of the shift depends on two things, viz : — 



1. The distance between the centre of the objective and the 

 centre of that part of it which may be in use. 



•_'. The distance of the plane* on which the object point lies 

 d the one in true focus. 



And with regard to the latter it is well to note that, whereas the 

 points on successive planes get shifted more and more the further 

 the} are from the one in true focus, all the points on any one 

 plai e g< t shifted by the same amount. 



: matter concerning the diffusion discs to which I 



All ] are r< arded as al right angles to the optic axis of the 



