JOURNAL OF THE 



April, 



in growing things. If the corners of these meshes be filled up, 

 the included circles will still keep to each other the relative 

 position of Brebisson's oblique quadrille. The diminution of the 

 round alveoli would not need to proceed far before the approxi- 

 mately rectangular mass of silex between the circles would be 

 about as large in diameter as the circles themselves. Under the 

 laws of optics, which we have already seen illustrated in print 

 No. 12, the tendency of approximately rectangular details is to 

 become more strictly so in the microscopical image. In Figure 

 I I have illustrated this by a geometric diagram of which one half 

 shows the square reticulation, and the other the resulting tessella- 

 tion of solid squares and round alveoli when the walls are thick- 

 ened and the corners filled up. It will be noticed that when the 



corners are so filled as to make the alveoli circular, the interspaces, 

 are approximately square, and,])eing solid, will be red or pink by 

 transmitted light when the alveoli are bluish-white. On the inner 

 side of the shell the thin circles, or ' eye-spots," are usually smaller 

 than on the outer side; the diffraction effect by transmission 

 of light will straighten the edges of the tessellated outline; the 

 squares will each have half the area of, and will be diagonal to^ 

 the original squares; and with their alternate colors we shall have 

 exactly the appearance which Mr. Smith describes, and which is 

 very well shown in prints Nos. i and 2, compared with No. 6. 



The peculiarity of the quincuncial arrangement of alveoli is 

 that when the circles crowd upon each other so as to become 

 polygons bounded by straight lines, they form hexagons instead 

 of squares, and even when they are circles in a continuous plate 

 of silex the hexagonal outline is a persistent ocular illusion. We 

 should expect, therefore, that the tessellated appearance with 



