N. E. GREEN OX THE SURFACE MARKINGS OF DIATOMS. 235 



nation by side light ; its surface was seen covered with shallow craters, 

 but when the lime light was shut off, and light thrown up from the 

 mirror, the usual hexagons alone could be seen, and no contrivance 

 would show the craters which were known to exist ; yet, when the 

 lime light was added, they were visible, but their situation was evi- 

 dently nearer the eye than the hexagonal forms which lay within 

 the surface, and could not be seen with the lime light alone. There 

 was also a series of minute beads lining the inner surface of the hexa- 

 gons, which could be seen only by transmitted light. Side light is, 

 therefore, powerless for interior forms, though particularly useful for 

 the exhibition of minute surface details. Each kind of illumination 

 has, therefore, a value of its own, which in no way interferes with that 

 of the other ; still, it must be admitted, that no examination of these 

 delicate objects can be considered complete till both methods have 

 been employed. 



I cannot conclude this paper without expressing the great satis- 

 faction experienced in the use of the German powers by Gundlach ; 

 the distance at which they stand from the object when it is in focus, 

 making them peculiarly valuable in this examination. I desire also 

 to return my thanks to Mr. Curties, who kindly placed several high 

 powers at my disposal. 



EXPLANATION OF PLATE XVII. 



Portion of Istlimia nervosa. 

 Portion of Bicldulphia. 

 Portion of Triceratium favus. 

 Portion of P. hippocampus. 

 Portion of Cocconeina. 

 Edge of fragment of P. angulatum. 

 Theory of structure of valve. 

 Isthmia enervis. 



[N.B. — The plate will be issued with the next number.] 



