TREE -RING BULLETIN
28
Annual Subscription, $1.50
Vol 7, No. 4
Single Copy, 50c
THE TREE -RING SOCIETY
President
Dr. A. E. Douglass
Secretary
Mr. Roy Lassetter
Treasurer
Mr. Edmund Schulman
Tree -Ring Laboratory
University of Arizona
Tucson, Arizona
THE TREE -RING BULLETIN
Editor -in -chief
Dr. A. E. Douglass
Managing Editor
Associate Editors:
Archaeology
Botany
Mr. E. Schulman
Mr. W. S. Stallings, Jr.
Dr. Charles J. Lyon
AUTHORS
The Tree -Ring Bulletin will publish papers resulting from original research in tree -rings
in relation to climatology, archaeology, and other fields. For reports of projects in tree -
ring dating, a tabular form as in Vol. 6, No. 1 is suggested. Until funds are available
authors will be requested to pay the cost of illustrations. Each contributor will be given
twenty -five copies of the Bulletin in which his article appears.
SUBSCRIBERS
All correspondence regarding subscriptions should be addressed to Mr. Edmund Schulman,
Tree -Ring Laboratory, University of Arizona, Tucson, Arizona.
NOTES ON THE TECHNIQUE OF TREE -RING ANALYSIS, II
A. E. DOUGLASS
CELL ILLUMINATION
Need for Best Views of Rings. In field and laboratory examination of individual rings one needs a method of applying momentary high magnifying
power to opaque surfaces without interrupting the tracing of a long sequence of rings often several hundred in number. The reason for this situa-
tion is quite evident to those who have attempted crossdating on a large
variety of specimens. Some woods have distinctive rings every three or
four years that give easy accuracy of dating. But certain relatively complacent specimens have only occasional rings that check accuracy. It is a
great aid if these check rings can be recognized at once even if they are
microscopic or possibly absent in part of their circuit.
Kerosene and Transverse Surfaces. The common transverse tree section,
cut with a saw, leaves too rough a surface for ring examination. Wetting
the surface with kerosene or even water helps a little to bring out the rings
but in sensitive sequences where some rings are very small or microscopic it
fails totally. A file greatly improves this surface but still leaves it entirely
unsatisfactory unless the wood is very hard. This is often the case with
central rings in well- seasoned wood such as well -preserved prehistoric
beams. At best this is not reliable in difficult cases of ring study. Such
surfaces have a great deal of broken down cell material which produces a
sheen making the rings hard to see. This sheen is overcome by the frequent application of water (as I found used in Sweden) or better by kerosene which in the dry climate of our Southwest lasts for hours and eventually disappears. Oils and heavier liquids may leave permanent marks.
These sawed or filed surfaces are much improved by a razor -cut radial
band for dating purposes. This was the form used by the author in hundreds of thousands of early measures which have rarely required correction.
However this transverse razor cut left some sheen on the surface upon
which kerosene always made improvement (Fig. 8) . This method had the
advantage and still has it of giving a chance of searching for a possible locally- absent ring around the whole circuit of the tree. Such locally- absent
APRIL, 1941
TREE -RING BULLETIN
29
rings are sometimes found most easily near a knot* where the ring series
is distorted and in places greatly enlarged as in Fig. 9.
The transverse cut surface can under certain conditions be brought 'to a
high visibility; for our purposes the extra time and labor are saved by
the diagonal cut. This cut is much easier to make than the transverse cut,
especially since the angle to the grain needs to be less than 45 degrees.
Kerosene or other moistening makes little improvement in this cut when
the illumination is correct. Nevertheless in field work, since this cut cannot readily be applied to stump -tops (except by a V -cut) , resort must be
made to a transverse razor -cut surface and a small bottle of kerosene with a
small swab should be taken along.
The V -Cut and its Illumination. Improvements in methods of reading
rings were made in 1926 when it was desired to carry a large number of
specimens to Flagstaff for examination during a Sabbatical year. In order
to avoid carrying a heavy load of sections, V -cuts were made from them.
This was done by two cuts of the saw an inch apart on the section surface
and slanted towards each other to meet at a depth of about an inch. Preparation of surfaces on the three sides of the V -cut led to a more efficient
method of viewing the rings. One side of the three was the original top
with the transverse section not easy to cut. The other two came together
down within the section. It was found very easy to cut good surfaces with
a razor blade on these slanting sides but it was not at first evident why the
surface produced was sometimes exceedingly efficient in showing the rings,
and sometimes would not show them at all or had to be turned at a very
high angle. Finally it was found that by bringing a light over the shoulder
to the specimen held in the hand while sitting upright, one obtained a good
probability of securing a satisfactory view of the rings. We know that those
two converging cuts were on the average slanting about 30 degrees from
the grain. When the long V -cut was held horizontally and near the eye
while one sat upright, with light coming in over the shoulder, the grain
was somewhere near the vertical and without twisting the specimen on its
long axis very much a good reflection was obtained on the surface of the
cells so far as they showed in the slanting cut. This was true whichever of
the two slant sides was used. So it nearly always gave a good view to the
eye at low power.
The Needs of Photography. But in this method of viewing the rings it
was very difficult to use a high power or to take photographs of a specimen
because the surface was rarely perpendicular to the line of sight and only
a small area could be brought into focus at one time. Thus good views
seemed to be accidental and further improvements were needed. With
higher magnification it was observed that these slant cuts leave the cell tips
exposed so that the tube -like structures give a brilliant reflection at the exposed ends of the larger open cells of the springwood that greatly increases
its visibility and leaves the latewood practically unchanged (Fig. 1, central
* Fig. 9 illustrates one of those common yet interesting experiences in crossdating.
The specimen, number BE -67, collected in 1923, was dated by skeleton plot in 1927
-the first by that method. Fig. 9B exhibits the triangular pin mark put on rings
A.D. 906 and 907 at that time showing 907 absent from the specimen. This locally-.
absent ring was only 37 years from the center.
Four years later we obtained an additional section. Now at the time of writing
this article, the class in dendrochronology needed a good ring record of early date
and we decided to take it from the extra section. After checking its dating, Mr.
Schulman told me that 907 was absent in the best radius but present near a knot at
another point. So the missing ring whose absence was assumed in 1927 was found
14 years later. One recalls that in other such cases as in this one the particular point
showing the highly deficient ring is in a length of reversed curvature near a knot.
TREE -RING BULLETIN
30'
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6
7
A :.:_: 98
part) This reflection made for great improvement in the ease of seeing
the rings.
Cell Structure. The cells as they fit together in the wood forming the
rings are very much like a bundle of glass tubes in which the earlywood
is represented by thin -walled tubes and the latewood by very thick -walled
tubes whose tangential diameter however is the same. So if a bunch of
cells is cut transversely an exact cross- section of each cell is obtained and
one can see the different thickness in the walls of earlywood and latewood
cells. If this cut end is held at a proper angle to the light it can give an
"end" reflection like a mirror but in such a position, of course, the cut surface is not at right angles to the line of sight, and so photography is hardly
practicable by direct reflection; visual examination in this position sometimes gives superb results. If now we cut our bundle of glass tubes at an
.
31
TREE -RING BULLETIN
APRIL, 1941
GRAIN
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Fig. 1. Cell illumination shows in bright central area by light coming from small
mirror above at correct angle to reflect on cells whose direction is shown by
headless pins.
Fig. 2. Glass model of cell structure, with light below; positions of the light in Figs.
2 and 3 are clarified in Fig. i 1 and related text.
Fig. 3. Same model with light coming from above (see Fig. 11) at illuminating
angle; bright reflected images appear inside the tube ends.
Fig. 4. Different sized objects between the light source and model.
Fig. 5. Enlargement x13 of reflected image from glass model showing reversal,
right for left.
Fig. 6. Similar enlargements x170- in the cell ends of pine wood, showing the same
reversal.
Fig. 7. Position of razor blade to cut at an angle of about 35 degrees to the grain;
this takes a long sliding stroke.
Fig. 8. Beneficial effect of kerosene on a transverse razor cut that leaves a relatively
rough surface; darker area moistened with kerosene shows the rings more
readily.
Fig. 9A One half of lens ring A. D. 907 found in 1941 microscopic but showing in reversed curvature near knot.
Fig. 9B The pin marks made 1927 to indicate that a ring should be there but could
not be found; each enlarged x10; note similar series of adjoining rings in
each part of Figure 9.
Fig. 10. Cut on "far" side of specimen and its relation to the grain.
Fig. 11. Relation of cut to grain and illumination; this is the "late" or bark end
of the ring growth and shows cut on far side.
Fig. 12. Relation of cut in mounted increment cores to grain, light, and mount.
angle between 30 and 45 degrees to the direction of the tubes, we find that
another kind of reflection comes into use. This could be called "lateral"
reflection because it is on the sides of the long needle- shaped cells.
Lateral Reflection Shown in Models. Fig. 2 shows the light below the
model so that the cut surface of the tubes points over the light and cannot reflect it to the eye.* Hence no reflection is seen from the internal or
external cylindrical surface of the tubes and therefore the area has a very
* This relation is somewhat clarified in Fig. 11 and its text. If one turns the model
one -quarter way around to view its right end, he sees it in the position in Fig. 11,
with the light however coming horizontally from the left of that.figure.
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TREE -RING BULLETIN
Vol 7, No. 4
low visibility. Fig. 3 has the light above so that the tubes point to some
place beneath the light, as represented in Fig. 11. They are then in a position to reflect the light upward where it can reach the eye or be viewed in
a microscope or photographed as done in Fig. 3, whether the reflection is on
the upper convex surface of each tube or lower concave surface. We have
found by testing both surfaces of pine wood cells that the lower concave surface is more commonly but not always the reflecting surface.
Enlargement of Cells Showing Reflection. Fig. 5 shows an enlargement
x13 of the lower end of one of the glass tubes in Fig. 3. At the tip of the
tube a reflection is seen on its inside curvature of two vertical objects, the
left one being smaller than the right. The originals of these are two bars
placed between the light and the cell model, the smaller object being on
the right as in Fig. 4. The reversal indicates reflection from a concave
surface. Fig. 6 shows an enlargement x].70 of the image in pine cells of the
same two unequal -sized objects shown in Fig. 4; here also the same reversal
appears as in Fig. 5 but the surfaces that cast the images are not as perfect
as the glass surfaces of Fig. 5.
Photographs of Rings Without and With Cell Reflection. We have photographed the different effects on pine rings as shown in Fig. 1. In this figure
the specimen is a V -cut with slant surface placed in a position of the model
in Fig. 2. The light coming from below to each end of the specimen gives
the wood a poor illumination as shown in the figure. Two pins inserted in
the wood show the direction of the cells. A tall black- headed pin at the
left gives a shadow indicating that the light is from below. The bright central area however is on the same piece of wood with reversed light from a
small mirror above while the direct lighting is intercepted by a paper shade
held by a pin below. Thus the central part of the wood, shaded from direct
rays of light, is illuminated by the reflected beam at the proper angle and
shows up brilliantly because of the reflection on the cell ends exposed in the
cut; this is verified by the headless pin (right) that gives through its length
a strong reflection of this indirect lighting, and throws a strong shadow.
Application of Cell Illumination. From the explanation given above we
can now understand what is taking place in the bright illumination, shown
in Fig. 1, which has proved so well adapted to the requirements of tree -ring
work. The various details of ring study discussed in the previous paper on
techniques (July, 1940 Bulletin) are based on the use of this form of illumination. The chronology photographs comprising our collection of a thousand or more exemplify the use of these methods.*
Arc Illumination. There is another feature of cell reflection which makes
the preceding technique more efficient. If we sit at a desk with a desk light
three or four feet away and hold a glass tube (or even a round pencil) in
hand below the eyes in such a position that it points below the light, we
can swing it up and down until it reflects the light on the surface of the tube.
Now by turning from side to side in the chair, we find that we continue to
get the reflection when the tube is pointing below the light at a long distance either right or left of the light. This would not be true of a series
of flat surfaces on the cell or tube, but with rounded surfaces this right and
left reflection is extended through a long arc. To express it in a different
way, we may hold the tube stationary and pointing to a place below the
light, then the light itself may be moved many degrees of angle to the right
and to the left and still give a reflection on the tube; or we may increase
the number of lights side by side so that as seen from the tube they occupy
* The photographic work was done for us by Mr. H. Faurest Davis as part of the cooperative work under the Carnegie Institution of Washington and the University of
Arizona.
APRIL, 1941
TREE -RING BULLETIN
33
an arc of the sky like a rainbow that forms a circle around the point opposite
the sun. This greatly increases the illumination. From this one sees the
the value in a very large source of light like a big north window close at
hand or a desk lamp placed as close as convenient to the specimen.
Shoulder Light. We have previously recommended that in a radial specimen representing a portion of a section, the diagonal cut should be made
on the far side when the sequence of the rings goes from left to right. In this
case the cut surface can be placed in a horizontal position and we can look
directly down upon it; the source of illumination is in front of us raised some
20 degrees or more above the plane of the cut surface. This is a perfect arrangement for photography as it produces a large flat area in a plane at right
angles to the optical axis of the camera or microscope.
In certain cases the diagonal cut for ring reading has been made on the
near side of the specimen. Such specimen mounted with late end to the
right does not reflect light that comes from in front. There are two ways
of imparting the illumination. One is to put a small mirror close to the
specimen on the observer's side of it so as to reflect light upon it from the
proper direction as in Fig. 1 above. Another way is to permit light to come
in past the observer and close on one or both sides of him to reach the part of
the specimen that is in view. This means that we make use of one or both
extreme ends of the arc mentioned in the description above; it is without
doubt the reason for frequent success in the "shoulder" method of illuminating specimens already described. Of course this use of a cut on the near
side does not cause an inconvenience in photography for then one simply
turns the specimen around and takes the photograph. It is chiefly in ring
measurement that this arc illumination becomes really useful.
Field Examination of Increment Cores. Increment cores when first taken
out of a tree are very fragile especially at the outer or sapwood end. This
is often the important end because the outer rings are apt to be much thinner than the inner ones as is common in the old age of trees. Hence a reconnaissance cut on the far side means cutting towards that fragile end (if the
operator is right- handed) which may thus be easily broken off. Hence to
save such specimens the cut sometimes needs to be made on the near side.
In this case the pull of the razor blade is towards the stronger part of the
core and the surfacing if done with care need not injure the specimen. The
cut should be made very shallow so that it can be deepened and changed a
little in direction afterwards. So out in the field if the cut is on the far side
it is only necessary to hold the core at a proper slant that will reflect the sky
or the sun or the sky near the sun while the observer faces in the same direction. If the cut is on the near side one turns his back to the sun and
uses the "shoulder" lighting, twisting the core until it gives the best reflection.
Mounting the Core to Give Proper Illumination. Either in cores or in
large wood specimens it is quite important that the angle between the cut
surface and the grain should be less than 45 degrees *; 30 to 40 degrees is good
and so we have put the proper angle at about 35 degrees. A horizontal position of the cut surface in relation to the mount is very convenient. So
one needs to mount cores with the grain inclined about 35 degrees above the
horizontal on the side towards the operator (see Fig. 12) . If the cut has been
made on the "near" side of the grain at time of collection, then of course
the grain must slant upwards on the side away from the operator. One can
usually see the direction of the grain on the outer end if the bark has fallen
off but the common way of telling the direction is by the increased cell re* This importance applies to pine, pinyon, and juniper but is less in Douglas fir on
account of the spiral reinforcement of the cells which diffuses the light.
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TREE -RING BULLETIN
Vol 7, No. 4
flection of light along the whole length of the core usually on each side where
the cells occupy a position tangent to the exterior curved surface.
This bright line is very easy to distinguish after once being seen'and shows
the way the cells are pointing. As the specimen lies in the mounting groove
before glueing, it can be turned until this bright line is between one third
and one half way from the top of the core down to the farther side. This is
marked in Fig. 12 where E is the part of the core directed upwards; H is at
the side of the core perhaps a little above the edge of the mount, 90 degrees
from E; F is at 45 degrees, in a direction exactly half -way between these
two. G is a point one third of the distance from E toward H. The bright
line along the core should be between F and G in order that a horizontal cut
may give the best illumination when viewed from a point vertically over it.
Twisted Cores. Occasionally one finds that the bright line along the core
indicates some rotation of the core in its structure along its length. Special
care is usually needed at the outer end where rings are apt to be smaller and
perhaps missing; so the directions just given are important for use at the
outer end allowing the other end to take its usual small change in rotation.
In cases of excessive twist in the core it may be better to place the bright
line on an average at the top throughout its length. In this way slanting
cuts may be made on one side or another. Such cases are rare and the general policy of arranging the slant as indicated has been found extremely
useful.
Tree -Ring Laboratory,
University of Arizona,
Tucson, Arizona.
April 8, 1941.