146 THE AMERICAN MONTHLY [May 



the Entomostraca and the large Rotifera and Protozoa, but 

 it is quite inadequate for the retention of Melosira, Peri- 

 diniura, Dinobryum.Raphidium, Euglena.aud other of the 

 smaller and frequently very abundant planktons. 



Micrography. — At a recent meeting of the Royal Mi- 

 croscopical Society, Dr. Measures exhibited an instrument 

 for micrography made by Reisa, having a new form of fine 

 adjustment, which admitted of the arm being made of any 

 length without throwing any weight upon the fine ad- 

 justment screw. The manner in which the speed of the 

 fine adjustment is reduced is most ingenious. The mo- 

 tion was extremely slow, being only 1625 in. for every 

 revolution of the screw. The application of the principle 

 of the endless screw is a novel way of slowing down the 

 fine adjustment. Among other advantages claimed for the 

 apparatus are the increased length of arm and the reduc- 

 tion of weight on the fine adjustment to one-fifth of that 

 which is usually put upon it. 



Condenser. — For histological and biological work, an 

 Abbe sub-stage condenser is indispensable. With the 

 condenser open, a cone of light, having an angle of diver- 

 gence of 120°, is brought to a focus ui)on the object. 

 Viewed with this intense light, the delicate contours of 

 transparent objects, which are made visible by differences 

 of refraction, are almost entirely lost, and the stained por- 

 tions of the specimens which would be, without the con- 

 denser, more or less concealed by the outlines of the un- 

 stained portions, stand out in the bold relief. This "iso- 

 lation of the stained image," as Koch terms it, is of great 

 value in histological study. An Abbe condenser should 

 be found in every histologist's laboratory. 



To Prepare Ground Glass. — Ground glass is a useful 

 adjunct to the laboratory appliances of microscopist and 

 microphotographer ; but it is not always possible to ob- 

 tain glass having a grain of suflBcieut fineness to be of 



