134 



PBACTICAL PHOTO-MICKOGBAPHY 



proposed camera, on which are fixed two metal rods or tubes 

 A, A (these should preferably be brass-cased iron tubes of about 

 three-quarters of an inch in diameter, as they are stiff er 

 and cheaper to purchase than rod or tube of solid brass). 



Any part of the apparatus that 

 requires to slide along these rods 

 should be supported on a piece 

 of wood which has two V-shaped 

 grooves (as C, Fig. 49) in line 

 along one side of it, and a plane 

 surface on the other side, as D. The 

 two V-grooves should be as far 

 apart as may be convenient, so that 

 a longer support is given to the 

 apparatus ; but in the case of the 

 focussing-screen of the camera, or 

 any similar part, then a distance 

 from end to end of five or six inches 

 is ample. When any part of the 

 apparatus sliding in this manner 

 requires to be clamped down, only 

 one screw is necessary, and this 

 should pass through the sliding- 

 plate as nearly as possible at the 

 centre E of a triangle which is formed 

 by the ends of the two V-groove 

 slides and the centre of the plane 

 point of support on the other side. 

 No part of this arrangement requires 

 to be made with any particular 

 accuracy. The two faces of the 

 V-grooves are preferably at right 

 angles to one another, so that 



when clamped down there is no tendency to jam, and also 

 to ensure that the movement is sufficiently free when the 

 clamping- screw is released. 



In placing any piece of apparatus on such a geometric slide 

 advantage should be taken of the method also set out by Lord 

 Kelvin of three points of support. If any piece of apparatus 

 resting on three points is set on a horizontal plane it then only 



FIG. 49. The Geometric Slide. 



