TANK AND AQUARIUM MICROSCOPES 267 



for some depth, and have some range in the water of a good-sized 

 tank. 



A beautiful instrument of this class has been devised by Mr. J. 

 W. Stephenson for the examination of living objects in an aquarium. 

 A brass bar is laid across the aquarium as shown in the woodcut i 



FIG. 208. Wrought iron magnified 250 diameters. 



FIG. 209. Ordinary steel magnified 250 diameters. 



(fig. 210). To adjust it to aquaria of different widths the support 

 on the left is made to slide along the bar, and it can be clamped at 

 any given point by the upper milled head. The milled head at the 

 side, by pressing on a loose plate, fastens the bar securely to the 

 aquarium. 



