114 THE AMERICAN MONTHLY [Apr 



pod, except that the envelope of the diatom endochrome 

 was much more visible, being highly refractive. I have 

 kept some of these Surirellas in this state for three years 

 virithout any great change. 



After about 2J years observation, fearing to loose these 

 remarkable diatoms, as the Pleurosigma were disappear- 

 ing, I cleaned up the two original cultures, obtaining 

 quite a quantity of the Surirella and Amphiprora, but 

 very few Pleurosigma. The surface of the glass in the 

 dishes that held them was removed, as if it had been 

 etched with acid. In many of the smaller cultures, the 

 sediment resembled coagulum ; was composed of minute 

 spore-like bodies held together by hyaline threads, some- 

 whatlike lichens, except that there was no such regular ar- 

 rangement. The spore bodies were much smaller and the 

 hyaline threads much finer and were not fungus. They 

 were of a nearly uniform length and when separated from 

 the mass were motile, doubling up and matting together 

 One of these masses is a dark purple, another a pale 

 pink, another olive green, and two or three are golden 

 brown, like diatoms, I have kept some of these, three 

 years without further development, and am unable to 

 tell what they are. 



To Ascertain Focal Leagth. 



E. M. NELSON. 

 [In Eng-lish Mechanic] 



The focal length of a microscope objective can be easily 

 and accurately determined in the following manner: — 

 Project the image of a stage micrometer by means of the 

 objective itself (no eyepiece or amplifier of any kind be- 

 ing used) on to a white paper or cardboard screen, placed 

 about six feet with low-power lenses up to a ^ in. and 

 three feet, with powers higher than a J inch, from the 

 lens. Determine the magnifying power by dividing the 



