58 University of California Publications in Agricultural Sciences [Vol. 3 



/. It will measure only a moderate range of sizes. 

 g. While very accurate for a hand instrument, it is not capable 

 of extreme precision. 

 The reason for the last two statements will be explained in the fol- 

 lowing pages. 



It is evident that it will meet quite well the requirements already 

 outlined. It fails at two points only — its moderate range might pre- 

 vent its use in very large timber, and its lack of absolute accuracy 

 may militate against it for very precise, scientific work. 



Most of the errors of such a dendrometer are easily kept negligible. 

 Of course at any considerable distance, small variations of diameter 

 are imperceptible and cannot be measured. Since the minimum visual 

 angle for normal eyes is one minute, two-tenths of an inch is the 

 smallest variation recognizable at a distance of fifty feet. This con- 

 sideration applies equally to all dendrometers which do not involve 

 telescopic observations, and the use of a telescope at once means a 

 heavy and awkward instrument. 



If the arm is not held at right angles to the direct line of sight, 

 the graduations on the arm will no longer measure the distance between 

 mirrors along the indirect sight line, nor will this distance agree with 

 the desired diameter. However, this error can never be large since, 

 unless the arm is in approximately the correct position, the second 

 mirror cannot be seen at all in the first, and to center its image 

 therein is an instinctive proceeding. For more precise work, however, 

 an additional aid may be afforded by vertical lines scratched into the 

 backing of each mirror at its exact center, which are to be brought 

 into apparent coincidence when the instrument is in use. An alter- 

 native method of obtaining the same result is to mask the fixed mirror 

 with dark paper until, at the most convenient distance from the eye, 

 the whole of the movable mirror can just be seen in it. The position 

 of such a mask is shown in figure 2, A. 



A rotation of the dendrometer about the axis of the arm will, of 

 course, raise or lower the indirect sight line running from the instru- 

 ment to the tree. Here again, however, unless the position is essen- 

 tially correct, the image of mirror 2 cannot be found in mirror 1. The 

 error resulting, moreover, is merely the amount of taper that occurs 

 between the points observed by the direct and indirect sight lines, 

 which is usually negligible. 



Of course, if the two lines of sight are not parallel, serious errors 

 will result. This depends on having the two mirrors parallel and is 



