DETAILS OF CURVE PRODUCTION. 55 



Preparation for measurement. These pieces were then examined to 

 find the longest sequences of clear and large rings, and guide-lines for 

 the subsequent identification and measurement were selected as nearly 

 as possible perpendicular to the rings. Such lines having been decided 

 on, two straight pencil lines, half an inch apart, were drawn and the 

 surface between these was " shaved." For this purpose, after the trial 

 of many other methods, a common safety-razor blade was clamped 

 to a short brass handle. With this very sharp blade the rough surface 

 of the wood is removed and the rings stand out very clear and distinct. 

 Besides the space between the lines, the region close outside is usually 

 shaved also for a preliminary trial at cross-identification, the final 

 marks being the only ones permitted between the guide-lines. 



The best light for observing the rings is a somewhat diffused light 

 coming sharply from the side. A light falling on the wood perpendicu- 

 larly is apt to be very poor, either for visual work or photography. 

 Light from each side must be tried, for there is often a great difference 

 between the two directions, due probably to the way hi which the 

 knife passed over the wood and bent the ragged edges of the cells. 

 In photographing, the colors involved and the result sought (%. e., to 

 show the red rings as black) require an ordinary plate and a blue color- 

 screen. 



When the surface is well prepared it is placed in a suitable light and 

 wet with kerosene applied by means of a bit of cotton on the end of a 

 small stick. This deadens the undesired details of the surface, and 

 brings the rings into greater prominence. The identified section is 

 now supported over the unknown and with watchmaker's glass in 

 eye and long needle in hand, the observer can make rapid comparison 

 and quickly put on the required marks. 



IDENTIFICATION OF RINGS. 



In the early Flagstaff work the rings were first numbered, beginning 

 at the outside without regard to the year in which they grew. But this 

 was found to add complexity and involve 'the use of a separate reduc- 

 tion from the provisional numbers to the true dates of the rings. 

 Accordingly the rings are dated at once as well as possible on some 

 selected section that gives promise of an accurate record. The identi- 

 fication mark is a pin-prick or very small hole placed on the last ring 

 of each decade. The middle year of each century has 2 pin-pricks and 

 the centuries are marked with 3; the 1,000-year mark is 4. Marks 

 found in error are "erased" by a scratch through them. 



After the selected section is dated with the greatest care not to over- 

 look or mistake any rings, others are dated by direct comparison with 

 it. The common practical test in such comparison is the relation of 

 width of a ring to its half-dozen near neighbors. For some unknown 

 reason, rings of diminished size seem to carry more individuality than 

 enlarged rings, and so they are usually picked out for cross-comparison. 



