232 



On the Examination of the Surface Markings of Diatoms 



BY THE OxY-CALCIUM LlGHT. 



By N. E. Green. 



(Read 23rd June, 1871.) 



My attention was called to the vexed question of the nature of 

 the markings on diatoms, by an observation in a pamphlet on the 

 use of Mr. Read's prism, in which the direction of light reflected 

 by it was compared to that of the sun on the surface of the moon. 

 The want of exactness in this comparison surprised me, for in using 

 the prism, the rays of light are directed through a transparent ob- 

 ject, whereas, in the case of the sun, they fall upon an opaque sur- 

 face. However, the simile had something attractive in it, and 

 knowing that minute markings on the moon's surface are best seen 

 when the direction of the light is tangential, as at half moon, I 

 determined to examine the surface markings of diatoms by side 

 light, employing oxygen in order to secure a sufficient amount of 

 illumination. The resolve was also made to avoid all mediums and 

 glass covers, so that nothing should interfere with direct vision. 

 Operations were commenced on a specimen of Isthmia nervosa, 

 which was cut out of a dry slide, and mounted on a stand to raise 

 it about an inch above the level of a stage, in order that a small 

 condenser might be. placed close to its side. The lime light was 

 then brought to the level of the object, at about six inches from 

 the stage, and a beam of intense light thrown across the surface. A 

 valve of Isthmia was examined first with a ^ by Ross, and after- 

 wards with a y 1 ^ by Gundlach. The surface rjresented a most re- 

 markable appearance, being studded with rows of small shallow 

 craters, the sharp edges of which projected slightly above, while 

 the centres seemed to be below the surface. A friend who was pre- 

 sent, exclaimed — " Why, 'tis like a nutmeg grater." The craters 

 on one portion were alternately large and small, and in another 

 part divided into petals like a flower. At the side of the valve the 



