INSTRUMENTS AND TECHNIQUE 39 



doing it completely a second time) is great, and so these errors of 

 identity are not discovered until a careful revision is made by the 

 writer. All this refers, of course, to the measurement of records care- 

 fully dated beforehand. 



Measuring directions — The best plan for preventing errors in 

 measuring is a written set of measuring directions, telling where to 

 begin and end, what rings, if any, to omit, which are small or micro- 

 scopic or absent, and where dangerous doubles occur. When a radial 

 sample is specially illuminated during measuring in order to see the 

 rings well in a telescope, marks and directions on the sample may 

 easily be overlooked, but a separate list on a paper at the side can be 

 followed with greater success. 



Other applications — Extensive experience with the ordinary filar 

 micrometer in astronomical work led to a design of this instrument 

 which could be used on a telescope for the repeated measurement of 

 the same distance, such as planetary diameters, separation of double 

 stars, and so forth. The box of the plotter was arranged to receive 

 on one side a bushing adapted to the slide-tube of a big telescope and 

 on the other a positive eyepiece. Close to the eyepiece is a plate 

 carrying a stationary thread, while another plate attached to the 

 carriage has the movable thread. The latter is first placed on the 

 left side of the planetary disk and the stationary thread on the right. 

 Then the lever-arm above described is pressed and the movable thread 

 carried to the right until it reaches the right edge of the disk when 

 the other is at the left edge. Thus the double diameter is measured. 

 This may be repeated as many times as desired before looking at the 

 record. A thread stretched along the tops of the columns will give the 

 mean value. This same method can be used in the measurement of 

 average seed diameters under the microscope or the sizes of grains of 

 sand or other objects under special study. 



LONGITUDINAL PLOTTER 



The measuring instruments so far described all require accurate 

 dating beforehand, for corrections are hard to enter after the ordinary 

 transverse plot has once been made. It happened that considerable 

 material came to the laboratory with groups of very small rings 

 which I did not have time to date, but at a time when there was 

 available the help of an assistant. It was therefore desirable for him 

 to put on a ring-count and make measures which I could correct at 

 my leisure. This was accomplished by the longitudinal plotter (Plate 

 5, B). It simply reproduces the spacings which exist on the wood on a 

 large scale that can be varied to suit the needs of the rings. It repro- 

 duces very rapidly and two independent records are placed side by side 

 on the long paper tape such as is used in adding machines. This is 

 called the longitudinal plot, or more briefly, the "long-plot." The 



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