268 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE 



Between the ends of the bar slides an arm carrying a sprung 

 socket, and the arm can be clamped at any given point of the bar. 

 Through the socket is passed a glass cylinder, cemented to a brass 

 collar at the upper end, and closed at the lower by a piece of cover- 

 glass. Into this cylinder is screwed the body-tube of the microscope 

 with eye-piece and objective, which are thus protected from the 

 water of the aquarium. The microscope is focussed by rack and 

 pinion (milled head just below the eye-piece), and in addition the 

 objective is screwed to a draw-tube, so that its position in the cylinder 

 may be approximately regulated. 

 is The arm of the socket is hinged to allow of the microscope being 



FIG. 210 



inclined in a plane parallel to the sides of the aquarium. The lower 

 milled head clamps the hinge at any desired inclination. 



The socket also rotates on the arm, so that the microscope can be 

 inclined in a plane parallel to the front of the aquarium. Thus any 

 point of the aquarium can be reached. 



As an adjunct, and admirable aid to the student of the tank and 

 pond, as well as a simple and easy means by which specific forms of 

 microscopic life may be found and readily taken, we call attention 

 to the tank microscope of Mr. C. Rousselet. It is illustrated in fig. 

 211 and scarcely needs further description. 



One of Zeiss's Steinheil aplanatic lenses, to which . we have 



