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the nearer trees, and thus commence encroachment Droughts

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^ /. 2). Dana on the Origin of Prairies. 803

do not destroy the ffrass, while they do the seedlings of the
trees ; and when, under such conditions, the old trees die, they
die without successors. The encroachment is the slow work of
centuries, because the standing trees are not injured by such an
undergrowth. They live out their yfe-time, if left to nature, and
only when they die does the meadow-surface complete itself.

•the forest, as has been explained, cannot ordinarily encroach
on the prairie. But if a prairie, through any change of circum-
stances, becomes permanently wet, so decidedly so as to weaken
the grass, buf not a region of permanent water, then the forest
has its chance for encroachment.

4. If moistness, then, is especially favorable to the growth of for-
ests^ a change in the moistness of a region occasioned by geological
events would be attended by a change in the adaptedness to such
growth. — The Champlain epoch of the Post-tertiary, when por-
tions of the continent over the higher latitudes were much de-
pressed, (in many parts 800 to 400 feet), and the more southern
much less so, and when the great upper terrace flats of our lakes
and rivers, often many miles in width, were made, was a time
of warmer climate over the continents than the present, as the
distribution of the terrestrial animal life of the era proves.*

It was, also, as the same terraces and the raised beaches prove,
an era of wider expanse of lakes and rivers over the land. It
was, therefore, in all probability, an era of moister climate
over these regions than now. This being so, it was an era emi-
nently favorable for the growth and extension of forests, when
trees would have taken possession of the dry land not under
water; that is, wherever it was not too dry, as even then was prob-
ably true of the far west. Now, after this Champlain epoch of de-
pression, an elevation of the land began, bringing the continent
up toward its present level, causing a change of climate to one
of greater coolness and dryness, draining extensive regions that
had been under water, and drying moist areas. Consequently,
there would have been, from the beginning of this change, a
tendency to a narrowing of the forest regions ; and, with such a
tendency, an actual narrowing would, in one region or another,
have begun.

As the various parts of the continent would have differed hy-
grometrically during the Champlain epoch in the same way as
now, the great dense forests of the continent, on this side of the
Eocky Mountains, would have then, as now, covered the eastern
portions, and the great treeless regions would have been beyond
the Mississippi. Any subsequent extension of the prairies that
accompanied the approach to the present era and condition
would, therefore, have been from the west toward the east.
The prairies would have gradually stretched their bare surfaces

* See the author's Manual of Oeologj, pages 547 to 667.

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804 /. D. Dana on the Origin of Prairies. ^

along the rivers, and made their winding way eastward among
forest-clad hills, wherever the dryness of the soil most favored
them. There would thus have been a slow yielding of the soil
to plants that could hold their places in spite of droughts.

At the same time the n^w-made lands about drying bogs,
lake shores, etc. would have taken their vegetation from the aa-
joining regions; the kind being determined by the dryness or
moistness of the region through the year ; by the nature of the
vegetation that was most accessible with its seeds; by the char-
acter of the soil ; or, if either the dryness of the "Climate, or its
moistness, were great, very largely by this one element alone,
the dryness determining the absence of trees in spite of all
other conditions, and the moistness their presence. Accordingly,
there are interminable forests in Eastern America, interminable
prairies on the slopes of the Rocky Mountains, and in the region
between an intermingling of forests and prairies.

The species of the Posb-tertiary trees, as may be concluded
from the observations on buried fragments, were probably iden-
tical for the most part, if not wholly, with those now existing,
(as is true also of the species of invertebrate animal life). Our
forests of modern trees date, beyond reasonable doubt, from this
remote Post-tertiary era. Their distribution began then, and
in accordance with the favoring conditions of the soil and cli-
mate. At the present time, we have the distribution of that era
as it has become after being modified by the effects of subse-
quent changes of level and of climate.

But the modifications from these causes are, even now, far
from being completed. As is well known, there are extensive
forest regions where the soil and climate are so dry that if the
trees were cut away they would not be able to re-establish
themselves. A conformity between the distribution of trees
and the climate is, consequently, only partially accomplished.
Any cause that destroys the trees of a region, be it tempest, fire,
or other agency, helps on this progress.

The prairies of the Willammet, and in part those of the Sac-
ramento, and also many east of the Rocky Mountains, have now
their "oak-openings," or surfaces scattered over with oaks, 15 to
20 or so, to an acre. But these trees may once have been in
thick groves, as they are now clustered near Feather River, in
the Sacramento region* (p. 296.)

A continent, or a large portion of it, cannot pass from a oon-
dition of greater to one of less moisture without ultimately ex-
hibiting it in the distribution of its forests and prairies.

It is, therefore, no objection to the theory here propounded,
that forests, when cut down, do not, if left to nature, always re-
sume their places, but give way to meadows. The times since
the ori^nal planting are changed, and meadows are on the in-
crease from natural causes, as well ajs through the agency of man.

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J. p. CoBke on the Spectroscope. 305

Art. XXXL — On the Construction of a Spectroscope with a num-
ber of prisms, by which tlie angle of minimum deviation for any
ray may be accurately measured and its position in the solar spec-
irum determined; by Josiah P. Cooke, Jr.

In an extract from a letter of the author published in thia
Journal, vol. xxxvi, p. 266, a method of adjusting the prisma
of a compound spectroscope was described, by which the ad-
justment for any portion of the spectrum could be obtained
with great rapidity and accuracy. A further study of the sub-
ject has shown that the method of adjustment then only briefly
described admits of the highest precision, and may be applied to
the exact measurement of the angle of minimum deviation of
the spectrum rays. It has been thus possible to apply the great
dispersive power of the large Cambridge spectroscope in deter-
mining the relative position of the various spectrum lines, and
to secure all the accuracy of which angular measurements are
capable. The value of such measurements is obvious, and with
the hope that this method will prove to be an assistance to in-
vestigators we propose to give in this paper a description of our
instrument and of the manner of using it.

The general construction of the instrument is shown in fig. 1.

The two telescopes are constructed in the usual way. The
telescope A, which we shall call the collimator, has an adjust-
able slit placed exactly at the focus of the object glass. The
small tube which carries the slit slides into the body of the tel-
escope, but a rack and pinion motion would be preferable, so
that when the focus is changed the slit will necessarily remain
vertical. The rays of light diverging from the slit and rendered
parallel by the object glass of the collimator next pass through

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306 J. P, Cooke on the Spectroscope,

a series of prisms adjusted around a conical wheel, which will
be soon described, and are then received by the telescope B.
The spectrum, which is formed at the focus of the object glass
of this telescope, is examined with eye-pieces of diflferent mag-
nifying powers in the usual way. The object glasses of the tel-
escopes are 2^ inches in diameter and have a focal length of 15J
inches. They rest in Y's and are provided with spirit levels and
adjusting screws. The frames which hold the telescopes are
supported on pivots turning in sockets at the ends of two arms
connected with the body of the instrument and may be clamped
in any position. The arm which carries the collimator is per-
manently attached to the main iron plate, but the arm, wnich
carries the observing telescope may be moved around the plate.
The details of the construction are shown in fig. 2, which
is a section made
through one of the
legs of the tripod
and the movable
arm, the telescope
with its frame and c,^
pivot having first f^wmm^
been removed from
ihe socket. (This
figure as well as
fig. 4 were drawn
to the scale of one
inch to a foot).
The parts are as
follows: H is an
iron tripod with
leveling screws ;
G is a hollow ma- . -.^^
hogany column ^w^'
with a conical cav-
ity at the top ; F is an iron cone which rests in the conical cav-
ity, supporting the whole body of the instrument and connected
by a long iron rod with a clamping screw beneath the tripod
By means of this arrangement the instrument may be turned as
a whole in the horizontal plane and the collimator directed to
the source of light. Above the iron cone and fastened to it se-
curely is the main circular plate of the instrument, which is 18
inches in diameter and | an inch in thickness. Near the outer
edge of this plate is inserted a band of silver, which is graduated
to 10" of arc. On the under part of the plate there is a neck
and Jit the center of the upper surface a socket, which are accu-
rately centered with each other and with the graduated limb.
Around the neck at E moves an iron collar, to which is attached
the arm bearing the observing telescope. This moves, therefbreu

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/. p. Cooke on the Spectroscope. 307

concentric with the graduated limb and bears a vernier by which
the angular motion may be determined, reading to 10". In
the socket of the lirst plate rests the pivot of a second plate,
C, which turns on the first and supports the prisms with the ad-
justing wheel A. The diameter of the upper plate is an inch
less than that of the lower plate, so as to expose the graduated
arc near the outer edge of the latter, and its upper surface is as
flat and even as possible. Rising from the center of the upper
plate there is a tall screw pivot of iron, B, on which turns a
conical wheel, made also of iron. By this motion the wheel
may be either raised or lowered. This wheel is an essential por-
tion of the instrument, and we will next consider the theory
of its use.

In the ordinary method of measuring the angle of minimum
deviation with a Babinet's goniometer the prism is placed on a
revolving plate at the center of the graduated circle, and so ad-
justed that the edge of the refracting angle shall be perpendicu-
lar to the plane of the circle, and its bisectrix parallel with a
diameter of the circle. The axis of the collimator and observ-
ing telescopes, moreover, being parallel with, a diameter of the
circle, it is evident that, when the prism and telescope are turned
into the position of minimum deviation, the vertex of the angle
of minimum deviation will coincide with the center of the
circle, and hence the arc intercepted between two radii of the
circle parallel to the axes of the two telescopes will be the
measure of the an^le required. This angle is practicall^r meas-
ured by first bringing into line of collimation the observing tel-
escope and collimator, so that the image of the slit at the end of
the collimator coincides with the vertical wire in the eye-piece
of the telescope. The position of the vernier on the graduated
arc is now noted. Then, having adjusted the prism, both the
prism and the telescope are turned to the position of minimum
deviation, and a coincidence established between the vertical
wire and one of the lines of the spectrum. The vernier is now
again read and the difierence between the two readings is the
angle of minimum deviation for the raj corresponding to that

It will be obvious however
from fig. 3, that it is not neces-
sary for the accuracy of this
measure either that the prism
should be placed at the center
of the circle, or that the axes
of the telescope should be par-
allel^ to one of its radii. K
only the bisectrix of the refract-
ing angle passes through the
center of the circle, the prism

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308 /. P. Cooke on the Spectroscope.

may be placed on the outer rim of the plate, and if only the ra-
dial arm, which carries the observing telescope, moves concen-
tric wiih the graduated arc, the axis of the telescopes themselves
may make any angle with the radius whatever. Let Op and
J be two radii of the graduated circle. Let Ap and Bq rep-
resent the axes of the two telescopes in collimation and making
any undetermined angles with the two radii. Place now a prism
at m and turn the radial arm Oq into the position Oy', but with-
out changing the inclination of the axis of the telescope to the
arm, and let BmB' be the angle of minimum deviation. Since
now the two triangles qsm and q^so are similar, it is evident
that the angle BwiB' is equal to the angle joy', and is therefore
measured by the arc intercepted between the radii Oq and Oq*.

In order now to apply this principle in the spectroscope the
glass prisms were mounted permanently in brass frames. The
frames rest on three brass pins which were adjusted by filing
until the refracting edge of the prism was perpendicular to the
iron place C fig. 2. Two brass pins were also attached to the
back of each prism and the lengths of these so adjusted that,
when the prisms are pushed against the conical wheel, the bi-
sectrix of the refracting angle snould coincide with a radius of
the wheel The last adjustment was made with the aid of a
guage cut from a sheet of tinned iron fitting at the same time
the periphery of the wheel and the face of the prism, which was
applied alternately on either side. The angle of minimum devi-
ation of the ray D was then measured for each prism in the fol-
lowing way : —

The prism having been placed on the plate with the pinsapplied
to the periphery of the wheel, the collimator was turned on its
pivot, and at the same time the plate C turned on its center,
until on applying the eye at the open slit and looking through
the object-glass towards the prism, the further back edge of tiie
prism, seen through the glass of the prism, appeared to coincide
with the nearer back edge seen directly. When this is the case,
it is evident that the rays of light which reach the eye from the
further back edge of the prism must pass through the glass par-
allel to the back edge of the prism, or, what amounts to the same
thing, perpendicular to the bisectrix of the refracting angle, and
when the light passes in this way the prism is at the angle of
minimum deviation. When the prism was thus placed the colli-
mator was turned slightly on its pivot until the axis of the teles-
cope prolonged passed through the center of the prism-face, and
was then securely clamped in this position. This adjustment may
be made with sufficient accuracy by the unassisted eye. The prism
having now been turned one side, the arm of the observing tel-
escope was turned on its center, and at the same time the tele-
scope turned on its pivot until it came into exact collimation with

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/. JP. Cooke on the Spectroscope. 300

the collimator. In order to facilitate this adjuBtment the tele-
scopes are provided with caps which cover the object-glasses of
the telescopes with the exception of a narrow vertical slit at the
center. The pivot of the observing telescope was next clamped
and the caps having been removed the image of the slit seen
through the observing telescope was brought into exact coinci-
dence with the vertical wire and the position of the vernier noted.
The prism was now brought back to its place by turning the up-
per plate of the instrument, and the observing telescope also
turned until the position of minimum deviation for the ray D
was attained and this well known double line brought to coincide
with the vertical wire. The limb was then again read, and the
difference of the two readings gave the angle of minimum devi-
ation for the prism.

In order to show that this method of measurement is perfectly
accurate, we give below the angles of minimum deviation of the
nine prisms of the Cambridge spectroscope measured as just de-
scribed, and in a parallel column the same angles measured in
the old way with the prisms at the center of the plate. It will
be seen that the differences are insignificant and within the
limits of error of observation :

Measured at Mensured on

center uf ploto. edge of plate.

No. 1, 29^ 31' 10" 29** 31' 10"

No. 2, 29^ 29' 10" 29° 29' 10"

No. 3; 29** 28' 10" 29* 28' 10"

No. 4, 29* 37' 0" 29* 86' 4(»" -20"

No. 5, 29* 28' 30" 29* 28' 40" +10"

No. 6, 29* 36' 30" 29* 36' 10" - 20"

No. 7, 29* 28' 10" 29* 28' 10"

No. 8, 29* 29' 30" 29* 29' 40" +10"

No. 9, 29* 28' 40" 29* 29' 40"

267* 37' 60" 267* 37' 30" -20"
Although the adjustments required may appear complicated,
they can be made in far less time than it has taken to describe
the method.

It is a well known fact that when a beam of homogeneous
light passes through a prism at the angle of minimum deviation
the incident and emerging pencils make the same angle with the
faces of the prism, at which they respectfully enter and leave
the glass. Hence a second prism like the first stands in the same
relation to the emerging beam in which the first stands to the
incident beam. If then, after the first prism has been adjusted
at the angle of minimum deviation a second prism be applied
against the wheel at the side of the first, by moving the prism
slightly to one side or the other, it will be easy to find a posi-
tion, in which this prism also is at the angle of minimum devia-
Am. Jouk. Scl— SBOOin) Sbritjs, Vol. XL,.Na. 130.— Not., 1865.

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J. P. Coohe on th€ Spectroscope.

Fig. 4.

lion, moving of course the prism on the plate without disturb*
ing the position of the plate itselE In like manner other prisnEis
may be added until the required number is obtained. (In the
Cambridge spectroscope there are nine glass prisms of 45®, as
shown in fig. 4.) This adjustment has only to be made, how-
ever, once for all, since
when the prisms are
adjusted they are fast-
ened to a very thin,
flexible brass ribbon,
which, passing through
a box at the back of
each prism, is there se-
cured by clamping
screws. The endfs of
this ribbon, moreover,
are attached to a small
brass drum, which, be-
ing wound up by an
ordinary clock spring,
draws the chain of
prisms tightly around
the conical wheel, a^jd
Jceeps them always in
place. By tracing the
path of a ray of homo-
geneous light through a
series of similar prisms,
as is shown in fig. 4,
it will be found that
the path of the ray within the prisms is always tangent to
the same circle, provided that it passes through all under the
conditions of least deviation. Assuming, then, that the dis-
tances between the prisms are invariable, as they must be when
the prisms are fastened to a brass ribbon as just described, it
will be evident from a moment's reflection that the greater the
refrangibility of the given ray the less must be the diameter of
the circle, around which the prisms should be arranged in order
that the ray may pass under the required conditions, and, know-
ing the dimensions of the prisms as well as the index of refrac-
tion and dispersive power of the glass, it is easy to calculate ap-
proximatively what the diameter should be in a given case.
The dimensions of the conical wheel A, fig. 2, were thus deter-
mined — the largest diameter, 9J inches, corresponding to the
extreme red, and the smallest diameter, bi inches corresponding
to the extreme violet rays of the solar spectrum. In order to
facilitate the adjustment a series of lines were engraved encir>

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/. p. Cooke on th$ Spectroscope, 811

cling the wheel at equal distances from each other and numbered
from 1 to, 12.

Having described the construction of the instrument it will
now be easy to understand the method of using it. Let us sup-
pose that the object is to measure the angle of minimum devia-
tion of the Wue ray of the strontium spectrum. By examining
any chart of the spectra of the chemical elements it will be
found that this line is situated roughly at somewhat less than
two-thirds of the distance from A to 11. If, then, we turn the
conical wheel until the pins of the prisms all rest against the
line marked 7, we shall have approximatively the true position.
We then adjust the collimator with reference to the first prism
exactly as before described for a single prism. Turning then
the upper plate so as to remove the prisms out of range, we
bring tlie observing telescope into collimation with the collima-
tor, as also before described, when on reading off the limb we
have the starting point for our measure. Vve next turn the
plate and move the telescope until the spectrum appears in the
field, and carefully bring tne bine line to coincide with the ver-
tical wire at the position of minimum deviation. We now
raise or lower the conical wheel and notice if in this way the
angular deviation is diminished, and leave the wheel in the po-
sition where the minimum is reached. It now only remains to
again read the limb when the difference of the two readings
subtracted from 860^ will give the angular deviation required.

When near the position of minimum deviation a large motion
of the conical wheel produces only a slight motion of the ima<jre,
so that after a table has been made giving the position of the
wheel for a few of the marked lines of the spectrum it is always
possible to bring the wheel at starting to the desired point. More-
over, the fact that when near the position of minimum deviation
the position of the image is affected so slightly by a small change
in tne position of the prisms, renders it possible to make all the
adjustments required with exceeding rapidity and accuracy.

In order to test the accuracy of our method we have made
several determinations of the minimum deviation of the line D,
and although between each determination the whole apparatus
was completely dismounted, the value obtained was in all cases
267° 37' 50".* It will be remembered that the sum of the angles
measured on each prism separately at the center of the plate, as
given on page 809, is precisely the same (267° 87' 50") and the
sum of those measured on the edge of the plate (267° 87' 80")
only differs from this by 20".

When it is not important to use absolutely the whole aper-
ture of the prisms it is not necessary to change the position of
the collimator in passing from one part of the spectrum to an*

* At the mean of the two lines*

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312 J. P, Cooke on the Spectroscope,

other. If we adjust the collimator as above described, when
the prisms rest against the middle circle on the wheel, the whole
spectrum cau be passed under review with great rapidity with-
out any further change of the collimator, and each point seen
under the condition of minimum deviation. When, however,
on account of the feebleness of the light it is important to use
the whole aperture of the prisms, a slight gain can be obtained
by readjusting the collimator at the extreme points of the spec-
trum. In making the measurements described in this paper, an
important advantage is gained in keeping the position of the col-
limator fixed; for if its position is changed, the point to which
the angular measurements are referred is changed also, and must
be determined anew. If however the arms, to which the tele-
scopes are attached, are so arranged with a sliding motion that
both the collimator and the observing telescope may be moved
parallel to themselves without altering their relative angular po-
sition, the whole aperture of the prisms may at any time be used,
and nevertheless all the measurements referred to the sami point
on the graduated limb.

Online LibraryUniversidad de Buenos Aires. Facultad de Derecho yThe American journal of science and arts → online text (page 83 of 100)