ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
77 
t are in phase 6. Then the two undulations reach every point of the 
dotted planes in the same phase. Hence markings seen by the ruling 
produced by those beams co-operating with rulings produced by other 
pairs of beams little sloped to u and u\ will appear to shift sidewards 
if we put the object a little out of focus. A case more frequently met 
with is represented in fig. 6. Here two pairs of undulations, u u' and 
v v', co-operate to produce one of the rulings by which the markings 
are seen. In the figure, the dotted lines sloping up to the left repre- 
Fig. 6. 
sent the planes over which u and u r are in the same phase ; and the 
dotted lines sloping up to the right, the same for v and v'. Hence 
if the objective be focused upon the horizontal plane through s, every- 
thing is in confusion, whereas on focusing a little farther out to p the 
ruling reappears, but now dark lines occur where bright ones were before. 
Thence follows : — 
Proposition 8. 
When the image of minute detail is produced by a triplet of beams , or 
by two pairs of beams , in each meridian , then the conditions are usually 
such , especially when the detail presents the appearance of round specks, 
that it will suddenly change from bright to dark , or vice versa, upon a slight 
change of focus ; and under special circumstances which are occasionally 
met with , more than one of these alternations may occur. 
And Proposition 9. 
The conditions are likely to be such , unless special precautions have been 
taken, that on a slight change of focus the minute detail upon the object 
vjill appear to shift its position relatively to the general position and broader 
features of the object. 
Experiments with Navicula lyra are described in illustration of the 
above points. 
The phenomenon of illusory coloration is strikingly exhibited by 
