290 THE MICROSCOPE AND ITS REVELATIONS. 



frustule is of discoidal shape, being composed of two delicately undulat- 

 ing valves, united by a hoop; so that, if the frustules remain in adhe- 

 sion, they would form a filament resembling that of Melosira (Fig. 178). 

 The regularity of the hexagonal areolation shown by its valves renders 

 them beautiful microscopic objects; in some species the areolae are small- 

 est near the centre, and gradually increase in size towards the margin ; 

 in others a few of the central areolse are the largest, and the rest are of 

 nearly uniform size; while in others, again, there are radiating lines 

 formed by areolae of a size different from the rest. Most of the species 

 are either marine, or are inhabitants of brackish water; when living they 

 are most commonly found adherent to Sea-weeds or Zoophytes; but when 

 dead, the valves fall as a sediment to the bottom of the water. In both 

 these conditions, they were found by Prof. J. Quekett in connection 

 with Zoophytes which had been brought home from Melville Island by 

 Sir E. Parry; and the species seemed to be identical with those of the 

 Eichmond earth. The investigations of Mr. J. W. Stephenson, 1 on 

 Coscinodiscus oculus Iridis show that the peculiar "eye-like" appear- 

 ance in the centre of each of its hexagonal areola arises from the inter- 

 mingling of the markings of two distinct layers, differing considerably 

 in structure; the markings of the lower layer being partially seen through 



Structure of siliceous valve of Coscinodiscus oculus Iridis: 1. Hexagonal areola of inner or 

 'eye-spot ' layer; 2. Areola of outer layer. 



those of the upper. By fracturing these Diatoms, Mr. Stephenson has 

 succeeded in separating portions of the two layers, so that each could 

 be examined singly. He has also mounted them in bisulphide of car- 

 bon, the refractive power of which is very high; and also in a solution 

 of phosphorus in bisulphide of carbon, which has a still higher refrac- 

 tive index. If we suppose a Diatom to be marked with convex depres- 

 sions, they would act as concave lenses in air, which is less refractive 

 than their own silex; but when such lenses are immersed in bisulphide 

 of carbon, or in the phosphorus solution, they would be converted into 

 convex lenses of the more refractive substance, and have their action in 

 air reversed. Analogous but opposite changes must take place, when 

 convex Diatom-lenses are viewed first in air, and then in the more re- 

 fractive media. Applying these and other tests to Coscinodiscus oculus 

 Iridis, Mr. Stephenson considers both layers to be composed of hexagons, 

 represented in Fig. 179, from drawings by Mr. Stewart. The upper 

 layer is much stronger and thicker than the lower one; and the frame- 

 work of its hexagons more readily exhibits its beaded appearance. The 



1 "Monthly Microscopical Journal," Vol. x. (1873), p. 1. 



/ 



