ZOOLOGY AND BOTANY, MICROSCOPY, ETO. 499 



object are to be pliotographed. The sensitive plate is now introduced 

 and exposed three or four times, the index being set at diflfcrcnt 

 points between the limits ; in this way three or four images are 

 superposed and form a complete picture. To obviate the reading of 

 angles, the circle is provided with two movable stops which can be 

 fixed at the limiting positions by means of screw clamps, so as to limit 

 the angular space through which the index can be turned, without 



Fifi. 84. 



the necessity of any reading. In practice it is best not to attempt 

 to obtain more than two or thx'ee successive impressions, since with 

 a greater number the figure becomes confused. It must be added 

 that the photographs are never so fine as those got from an object 

 which can be completely photographed by a single exposure." 



Beck's Demonstration Microscope. — The instrument shown in 

 fig. 85 was devised by the late Mr. R. Beck for the purpose of securing 

 delicate objects against injury at soirees and similar exhibitions. 



The special point consists in inclosing the Microscope in a box 7 in. 

 X 6^ in, X 19 in,, into which it is locked, there being doors on either 

 side. The binocular body is fixed to the front of the box by a bar, 

 and also to the top, and the draw-tubes can be extended by the milled 

 head at the side. 



At the back of the box is a horizontal pivot on which turns a lever- 

 piece with two equal arms. The stage slides on the lower arm, to 

 which it can be clamped. This enables the object to be placed ap- 

 proximately in focus. For a fine adjustment the top of the upper arm 

 can be pressed forward against a spring by the milled head at the 

 back, the stage being then slightly tilted. The pivot on which the 

 lever-piece turns can also be raised or lowered and clamped. This we 

 presume was intended to provide for a more extended motion of the 

 stage than could be obtained by sliding it on the lower arm of the 

 lever. 



The lamp is placed on a bracket in front, which is attached to a 

 vertical sliding piece having a circular aperture which admits the light 



