514 Dr. G. J. Stoney on Microscopic Vision. 



u f —8L case which occasionally happens — then if we put the 

 object a little out of focus these markings will appear to travel 



Fig. 1. 



Image plane. 



along the inclined dotted lines and will appear to shift side- 

 ways if the dotted lines are inclined, which they will he if, 

 as usually happens, the angles of incidence of u and u f are 

 unequal. This shifting need not extend to the larger features 

 of the object, since they are seen by light that is quite 

 differently circumstanced. In the case represented by the 

 figure the markings that are seen do not pass from bright to 

 dark. Their definition merely fades continuously into a 

 haze while they are being put out of focus inwards or out- 

 wards. 



But a case which is much more frequently met with is 

 represented by fig. 2. Here two pairs of undulations u, v! 

 and v, t/, both in the same meridian plane, cooperate to pro- 

 duce one of the rulings by which the markings are seen*. To 

 avoid complication the wave-surfaces are not represented in 

 this figure, but the dotted lines sloping up to the left repre- 

 sent, as in fig. 1, the planes over which u and u l are in the 

 same phase. The lines sloping up to the right represent the 

 same for v and v 1 . Hence if the objective be focussed upon 

 the horizontal plane through s everything is in confusion 

 and the image disappears, whereas on removing the focus a 

 little farther out to p the ruling reappears, but now dark 



* The case where v and u coalesce and form the dioptric beam, 

 while u and v' are diffracted beams, is that most frequently met with. 



