1917] Bruce: A New .Dendromt t> r 57 



above outlined. The following pages describe an instrument based on 

 a somewhat different principle from those previously devised, which 

 will be seen in a large measure to meet these requirements. 



It consists essentially of a straight arm upon which are mounted 

 two small mirrors, both at an angle of 45 degrees with the axis of the 

 arm, parallel to each other and facing in opposite directions (see 

 fig. 1). One mirror is fixed at one end of the arm, while the other 

 is mounted on a slide which travels along the arm. Graduations 

 permit a direct reading of the distance between the mirrors. 



The principle is indicated by figure 1 which shows the relative 

 position, as seen from above, of tree, observer's eye, and of the instru- 

 ment when in use. It will readily be seen that the instrument is 

 closely akin to the ordinary calipers in principle, except that for the 

 parallel fixed and movable arms of the calipers are substituted two 

 parallel lines of sight. The direct line of sight passes just above the 

 upper edge of the fixed mirror from eye to one edge of the tree, while 

 the indirect line of sight is reflected in each of the two mirrors to the 

 other edge of the tree. That the two lines of sight are parallel and 

 hence that the distance between the mirrors is equal to the diameter 

 of the tree is too self-evident to demand geometrical demonstration. 



In use the observer holds the dendrometer arm horizontal (if the 

 tree is in the normal vertical position) with one of the mirrors in line 

 between his eye and the left-hand edge of the tree at the point to be 

 measured. He then catches the reflection of the second mirror in the 

 first, thus bringing the arm into a line perpendicular to the lint 1 from 

 eye to tree. By sliding the second mirror in or out, the right-hand 

 edge of the tree will become visible in it. The adjustment is now 

 continued until the left-hand edge as seen directly and just above the 

 fixed mirror, and the right-hand edge as seen indirectly through the 

 two mirrors, are in a straight line, one immediately above the other. 

 The distance between the mirrors as read from the graduations on the 

 arm is then the required diameter. 



The advantages and disadvantages of the instrument are evident. 



a. It is direct reading. 



b. The distance from the observer to the point observed docs no1 



have to be determined. 



c. As a result, the instrument is rapid in use. 



d. It may be set for a given diameter, regardless of distance. 



e. It is light in weight and of convenient shape for carrying; 



it is more portable than a pair of calipers of the same range. 



