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University of California Publications in Agricultural Sciences [Vol. 3 



but to the nearest .2 inch. With a hand instrument of the common 

 type which involves the measurement of the angle between two sight 

 lines it is difficult to provide for an accurate reading closer than to 

 the nearest 10 minutes. This means that at the same distance 1.7 

 inches would be the minimum recognizable difference in diameter. 



Figure 2 shows the details of construction. 1 a-a, of A, B, and C, 

 is the straight arm which is made of a casting of aluminum alloy. 

 The straight edge is the back surface of the slot which is recessed into 

 the upper surface of the arm, as is best seen in the cross-section. This 

 cross-section is perhaps unnecessarily heavy, but was so designed to 

 insure as perfect a straight edge as possible. In this slot travels the 

 slide d shown in detail in D, E and F, which are drawn to twice the 

 scale of A, B and G. This slide is equipped with two springs, h and i, 

 which hold it against the back and upper surfaces of the slot. Upon 



Fig. 3 



it is mounted the mirror platform, and mirror e turned to an angle 

 of 45 degrees to the axis of the slide. At the end of the arm a second 

 fixed mirror platform, b, is mounted on which is the second mirror, 

 /', which can be adjusted by the two opposed adjusting screws, g-g. 

 This mirror is shown with both center line and mask, though both are 

 hardly necessary. The scale is readily seen in A. This is read by 

 means of the small arrow engraved on the side of c, as shown in both 

 A and E. In A the reading, for example, is 12. 2-. 



If the weight of the instrument, slightly less than 27 ounces, is 

 found objectionable, it would probably be safe to lighten materially 

 the cross-section of the arm by reducing both the depth of the down- 

 ward projecting ribs and the thickness of the lateral walls. The 

 mirrors also, as shown, are very generous in size, and might be 

 reduced to about two-thirds the indicated dimensions without intro- 

 ducing any serious difficulties through restricting the field of vision. 



i To Mr. V. Arntzen of the Civil Engineering Laboratory of the University 

 of California, credit is due for the major part of the detail of design. 



