334 Professor J. D. Everett on a 



one degree of freedom, and its length is a definite multiple of 

 the longitudinal diagonal of a rhombus. 



The joints are arranged in three rows, one down the middle 

 and one along each edge, and the distance from joint to joint 

 in any tow is equal to this longitudinal diagonal. This 

 common distance can be varied between very wide limits by 

 pulling out or pushing in the frame, and we have thus a means 

 of dividing an arbitrary length into any number of equal 

 parts. # I utilize only the middle row for this purpose, and 

 utilize it only or chiefly for bisecting a variable distance. 



The pins on which the middle joints turn are continued 

 upwards, as shown in fig. 2, to serve as supports for clips 

 holding the object, the lens, and the screen. The lens is 



Fig. 2. 



Elevation. 



mounted on the centre pin, and the object and screen usually 

 on the two end pins, as in fig. 2. In order to avoid flexi- 

 bility, the clips are made short, and the pins, on which they 

 are held by screws, rise only 1^ inch above the frame. The 

 base of each pin is a substantial disk (see fig. 3) which rests 

 upon the table ; all the pins, not only in the middle row, but 

 also in the two outside rows, terminate in such disks, which 

 serve as the feet of the instrument, and slide upon the table- 

 when the frame is expanded or contracted. The pins are 

 of brass J inch in diameter, and the bars are of ^-inch 

 mahogany, f inch wide, and 13 inches in gross length. 

 There are 18 of them, as shown in fig. 1. There are 9 

 pins in the middle row ; and when the object and screen 

 are on the two end pins, the distance between them is 

 divided by the other pins into 8 equal parts, any two of 

 which should together make up the focal length. The 

 unused pins are the most convenient handles for manipulating 

 the frame. 



The screen may conveniently be a piece of white card a 

 little larger than a post-card, and a square of wire-gauze 

 about half as big may be used as the object ; but a still 

 better ' object ' is a cross of threads stretched across a square 

 hole in a card. The light which passes through the square 

 hole is very conspicuous on the screen before the correct 

 distance is approached, whereas the shadow of the wire 



