38 THE AMERICAN MONTHLY [Mar. 



double color discs having a central spot of one color sur- 

 rounded by a ring of a strongly contrasted or comple- 

 mentary color. He prefers a red centre and a green 

 periphery. By means of the iris diaphragm, the rela- 

 tive proportions of the two colors are so regulated that 

 in looking through the lenses the light appears to be of a 

 neutral tint. This arrangement is suitable for use with 

 high power objectives. 



Recent Diatom Discoveries. 



A long-shaped aperture in the nodule of Navicula 

 rhomboides has been found by Mr. Nelson using^Powell's 

 apochromatic adjustible condenser. Pipes and a central 

 spot in the nodule had been before seen but no aperture 

 in so small a species of Navicula as the rhomboides. 



In the diatom Biddulphia elaborata the termination of 

 the stalk is called "the rose of the diatomic watering 

 pot." Attached to the oval periphery of the valve is a rim 

 only .00041 of an inch high. The general appearance of 

 the valve can be compared to an oval tea-tray having a 

 convex mound in the centre as high as the rim and a pipe 

 with a watering-pot rose top rising up a little distance 

 from the ends of its longer axis. The close-set papillse 

 which are small pipes analogous to the perforation in the 

 nodule of a Navicula rise from the centre of a crater 

 which is at the top of an elevation in the middle of the 

 valve. The edge of the crater is level with the top of 

 the rim round the periphery of the valve. 



The ridges radiating from the center of the valve be- 

 tween the rows of large areolaiions are caused by a thick- 

 ening of the silex, being located in the thinner part of 

 the silex. On the thick ridges between the rows of are- 

 olations are intercostal dots. They are very irregular 

 and many are missing. 



