Edmund C. (Edmund Clark) Sanford.

A course in experimental psychology online

. (page 13 of 33)
Online LibraryEdmund C. (Edmund Clark) SanfordA course in experimental psychology → online text (page 13 of 33)
Font size
QR-code for this ebook

portion of the vertical diagram is reflected from the upper
surface of the plate, and is therefore uncolored. 2 The mix-
ture of the two gives a light green field. For simplicity,
we may assume that no light comes from the black portions
of the diagram. Then in the portion of the light green

1 Any black spot will answer. For this experiment diagrams made up of sets
of heavy concentric black rings, lines a quarter of an inch wide, separated by
white rings of triple width, give an excellent effect. The diameters should be so
chosen that a black ring on the horizontal diagram shall correspond to a white
one on the vertical and vice versa, and shall appear to lie in the midst of the
white when the diagrams are combined in the way described above. A pair of
diagrams made up of parallel black bars, a quarter of an inch wide, separated
by quarter inch spaces, and so placed in the instrument that they give a checker-
board pattern when combined, are useful for keeping in the field a true black
with which the changed colors can be compared.

! Asa matter of fact, a small portion is also reflected from the lower surface
of the glass, and contributes a minute amount of green.

Digitized by



field corresponding to the black of the vertical diagram, the
white component will be wanting and the green will appear
undiluted ; in the portion corresponding to the black of the
horizontal diagram, the green component will be wanting
and the faint white (i. e., gray) should appear by itself.
It does not, however, because of the contrast color induced
upon it. As a matter of fact, the black portions are not
absolutely black ; the small amount of light that comes
from them tends on one hand to make the green image (im-
age of the black of the vertical diagram) a little whiter, and
on the other hand to counteract the contrast in the purple
image by adding to it a little green. Try the experiment
with other glasses than green.

Another form of the mirror contrast experiment is as
follows. Place a mirror where the sky or a white surface
of some kind will be seen reflected in it. Lay upon its sur-
face a plate of colored glass (green for example), and hold
a little way above it a narrow strip of black cardboard or a
pencil. Two images will be seen : one a vivid green, the
other a complementary purple. The green image belongs
to the surface reflection of the colored glass, as may be
proved by observing that when the strip of cardboard
touches the surface, the green image touches it also. The
purple image belongs to the reflection from the back of
the mirror. It is easy, by substituting a gray strip for the
black, to show that contrast can suppress a weaker objective
color actually present. 1

c. Meyer's Experiment. Lay on a large colored field a
small piece of gray or even black paper (e.g., 1 cm. wide by
2 cm. long), and cover the whole with a piece of semi-
transparent white paper of the same size as the colored
field. The contrast color will appear on the gray paper.

1 For fuller explanation with diagram, see American Journal of Psychology \
V., 1892-03, 407, and von Bezold, 154 f.

Digitized by



If thin tissue paper is used, more than one thickness may
be needed for the best result. Paper mats, woven one way
of gray paper and the other of colored, show this contrast
beautifully. They may easily be made from kindergarten

d. Mixed Contrasts with the Color-mixer. Disks made on
the pattern of the cut at the left show beautiful contrasting

grays. The disk used in Ex. 145 c shows a longer series,
but requires a more rapid rate of rotation. The same can
be shown also by laying a number of small sheets of tissue
paper over one another in such a way that they partially
overlap, making a portion where there is but a single thick- '
ness, and next it a portion where there are two thicknesses,
and next that again one of three thicknesses, and so on.
When the whole is held up to the light, the contrasts of
adjacent portions are very easily seen.

Contrast colors can be shown finely with disks like that
in the cut at the right, in which the shaded portions repre-
sent color, the black portions, black, and the white, white.
A little care is necessary in fixing the proportions of the
color to white and black in the disks, but in general the

Digitized by



brightness of the gray should be about that of the color.
When the contrast color has been satisfactorily obtained,
bring near it a piece of white cardboard (e.g., 3x5 in.), so
held with reference to the source of light that it appears
about as bright as the contrast ring. Hold the card so that
its shadow does not. fall on the disk, or at least is out of
sight. Notice the retreat of the contrast color from its
edges. On such experiments as this much stress is laid by
Helmholtz and the supporters of the psychological explana-
tion of contrast.

Contrasts with two colors at once can be shown by mak-
ing the inner portion of the colored sectors of one color, the
outer portion of another. A temporary disk for showing
contrast effects may be arranged by putting on the spindle
of the color-mixer first a large colored disk (e.g., 20 em.
in diameter), then smaller combined disks of black and
white (e.g., 12 cm. in diameter), and finally a still smaller
colored disk (e.g., 10 cm. in diameter).

Helmholtz, A, 542 ff., Fr. 515-546 (392-417); Hering, E; Aubert,
496 ff., 546 ff.; von Bezold, 144-171; Rood, 241-272; Mayer.

For particular experiments, see the following: on a (second part),
von Bezold, B, 153-154; on b (second part), Dove; on c, Meyer.

For quantitative measurements of contrast in grays, see Ebbing-
haus, B ; Lehmann; and Kirschmann, D.

153. Some of the Conditions that Influence Contrast.

a. Contrasts are stronger when the colors are near to-
gether. L^y a bit of white paper on a black surface, e.g.,
a piece of black velvet, and notice that the paper is whiter
and the velvet blacker near the margin of the paper than
elsewhere, notwithstanding that the eye moves about freely.
This has received the name of " Marginal contrast " (Rand-

On a piece of gray paper, the size of a letter-sheet, lay two
strips of colored paper close side by side (e.g., pieces of

Digitized by




red and yellow or of green and blue, 1 cm. wide by 4 cm.
long). Below them to the right and left, as far apart as the
paper will permit, lay two other strips of the same size and
color, red on the red side of the former pair, yellow on the
yellow side. Notice the effect of the difference in distance
on the contrasting pairs. Contrast of this sort is at a maxi-
mum when one color entirely surrounds the other.

b. Effect of size. When the area of the inducing color is
large and that of the induced color is small, the contrast
is shown chiefly on the latter ; when the two areas are of
about equal size, as in a above, the effect is mutual. Try
with large and small bits of paper upon a colored field.

c. Borders and lines of demarcation that separate the
contrasting areas tend to lessen the effect by excluding mar-
ginal contrast ; and (since the eye tends to move along
rather than across strongly marked lines), by hindering such
motions of the eye as would bring about successive contrast.
Eepeat Ex. 152 c, using two slips of gray paper 5 mm. wide
by 2 cm. long, and substituting a piece of moderately trans-
parent letter-paper for the tissue paper. When the contrast
color has been observed, trace the outline of one of the
slips with a fine ink line upon the paper that covers it, and
notice that the color nearly or quite vanishes. A disk like
that in the cut accompanying Ex. 152 d, when provided with
a second contrast ring, marked off on both its edges with a
firm black line, shows a weakening of the induced color in
the bordered ring.

This experiment and others like it play an important part
in the psychological, as opposed to the physiological, expla-
nation of simultaneous contrast; see Helmholtz, -4, 543 ff.,
559 f., Fr. 533 f., 539, 542, (406 f., 411, 414). Such a black
border will, however, also make a weak objective color
invisible. x

d. Saturation. Contrast effects are generally most strik-

Digitized by



ing with little saturated colors. Compare the effect of
increasing, decreasing, and extinguishing the second non-
colored light in the colored shadow experiments. It is
necessary, however, to see to it that reflected light from the
walls and surrounding objects does not complicate the ex-

Compare the intensity of the contrasts in Meyer's experi-
ment (Ex. 152 c) before and after the application of the
tissue paper. Notice also the part played by the white
light mixed with the colored light in the mirror contrast
experiments above. Try the effect of introducing white or
black or both into the largest and smallest disks in the
arrangement mentioned at the end of Ex. 152. Powerful
contrasts with the most saturated colors can be observed,
however, when the proper conditions are fulfilled.

e. Colors induced upon gray fields are stronger when the
gray has about the same brightness as the inducing color.
Repeat Meyer's experiment, using white paper instead of
the gray or black. With the three disk arrangement try
the effect of making the intermediate disk all white and all
black. Rood finds that grays slightly darker than the
inducing color are advantageous when the inducing color
is red, orange, or yellow, and slightly lighter when the
inducing color is green, blue, violet, or purple.

On conditions in general, see Helmholtz, A, 540-541, Fr. 513->514,
(390-391), Kirschmann, D. In Hering, IF, will also be found much on
the effect of various conditions. On 6, Exner, B. On c, Helmholtz,
A, 546-547, Fr. 539-542 (411-414). On d, Helmholtz, A, Fr. 523-524
(399-400). On e, Rood, A, 261.

- 154. The Halo or Lichthof of Hering. Contrast is often
to be seen in negative after-images. That observed in after-
images of white objects on a dark ground has been adduced
by Hering as an argument against the psychological expla-
nation of contrast. Some of the simpler experiments are

Digitized by



as follows ; for his development of them consult Hering, A.

a. Lay a half inch square of white paper on a large sheet
of black cardboard (or better of black velvet), and put a
small dot at its centre. Stare with unmoved eyes at the
dot for from 15 to 30 seconds or more, then close and cover
the eyes. There will then be seen, neglecting incidental
color effects, the dark after-image of the paper surrounded
by a halo of light, brightest next the paper and gradually
falling off in brilliancy toward the periphery. This is ex-
plained on the psychological theory as due to contrast with
the deep black of the after-image of the square. When,
however, the converse of the experiment is properly made
(a black square on a white ground), the dark halo which
would be expected by contrast is not found, though the
after-image of the black square is very bright.

b. Lay two white squares side by side two or three milli-
meters apart on the dark ground and between them a
minute clipping of paper for a fixation point. Secure the
after-images as before. The halos of the two squares
coincide in the narrow space between and give a much
brighter band in the after-image. Under favorable circum-
stances this bright band may remain visible while the after-
images of the squares themselves are temporarily invisible.
In both these experiments it is better to use both eyes than a
single one. The explanation of the halo as a matter of false
judgment, especially in the last mentioned case, is not easy.

Hering, A.

155. Simultaneous Contrast with Colored Shadows. The
effects of simultaneous contrast are almost always lost in the
more powerful ones of successive contrast. The first requi-
site, therefore, of an experiment on the first, is the exclusion
of the second. This is not difficult for colored shadows.

a. Place a good-sized piece of white paper on a table in
such a position that it may be illuminated at the same

Digitized by



time from a window (if the day is overcast) and from a gas-
jet. Set upon it a small block or other object (about 5 cm.
by 10 cm. in size) ; something black in color is best. Light
the gas and observe the two shadows, one cast by the light
from the window, the other by the gas. The first will
appear yellowish, the second clearly blue. 1 Adjust the dis-
tance and position of the block with reference to the light
so that the shadows shall appear about equally dark, and
the blue shadow shall be as sharply bounded as possible,
and for that purpose it is well to have the shadow cast by
the edge rather than the flat side of the flame. The color
of the yellowish shadow is objective and due to the yellow
of the gas-flame, that of the blue is due to the contrast, but
largely, as yet, to successive contrast. Put a dot in the
centre of the blue shadow, to serve as a fixation-point, and
another on the edge. Fasten a paper tube (preferably
blackened inside) so that it can easily be shifted from one
dot to the other. Cut off the gas-light by holding a card
between it and the block ; adjust the tube so that the dot in
the middle of the shadow may be fixated without any of
the field outside of the shadow being seen. Wait until all
of the blue has disappeared from the shadow, and then,
still looking through the tube, remove the card. The field
remains entirely unchanged and appears, as before, a color-
less gray. The former blue color is thus shown to be sub-
jective and due to contrast with the yellow lighted area in
which it lies.

1 This setting of the experiment succeeds best when the daylight is weak, as,
for example, just before the lights are usually lighted in the evening. If the ex-
periment is to be made in broad day, the light must be reduced by curtains or
otherwise; if at night, there must be two lights, one corresponding to the win.
dow and one to the gas, and the latter must shine through a pane of colored glass.
If yellow glass is used, the colors will be the same as those in this experiment, the
free flame taking the place of the daylight. If the sky is dear, its light is itself
blue, and would complicate the experiment somewhat. Its light may, however,
be passed through colored glass or gelatine, but then the orange color of the
gas-light must be regarded.

Digitized by



b. Cut off the gas-light again and adjust the tube so that
the dot in the edge of the shadow may be fixated. Taking
great care not to move the eye, withdraw the card. The
part of the field of the tube filled by the shadow will ap-
pear bluish, that of the remainder reddish yellow. After
a little time of steady fixation, cut off the gas-light once
more and observe the instant reversal of the colors. The
shadow now appears in reddish yellow, the rest of the field
blue. The color of the shadow, both before and after the
final interposition of the card, is due to simultaneous con-
trast, in the first case with the reddish yellow light, and in
the second with its after-image.

Helmholtz and' his supporters explain all cases of simul-
taneous contrast as errors of judgment ; in the case of the
colored shadow, for example, we mistake the yellow of the
gas-lighted field for white, and consequently find the shadow
which is really gray to be bluish. In the case of this par-
ticular experiment, Hering and Delabarre have shown this
psychological explanation unnecessary and a physiological
one all sufficient, and Hering has done the same for other
forms of experiments.

On simultaneous contrast in general, see Helmholtz, -4, 542 ff.,
Fr. 515-547 (392-418); Hering, A and E. On colored shadows see
Helmholtz, A, 551-553, Fr. 517-519 (394-396); Hering, E ; Delabarre.

On Helmholtz's theory see Helmholtz, A, 543 ff., Fr. 516, 533-538
(392, 407-411); Hering, E\ Rood, A, 252 if.; von Bezold, J5, 146 ff.

For quantitative measurements of simultaneous contrast under
various conditions, see Kirschmann, D.

- 156. Simultaneous Contrast. Hering's Binocular Method.
a. Set a red glass in the right frame of the binocular
color-mixer, a blue glass in the left. Look fixedly through
the colored glasses at the cork ball below, bringing the eyes
cjose to the glasses and the nose between them. Adjust the
side screens till the white ground below appears in a uni-

Digitized by



form light violet from the binocular mixture of the red and
blue (see Ex. 167). The narrow strip of black paper on the
white is seen double, the right hand image bluish, the left

b. The possibility of successive contrast, however, is not
yet excluded. Lay a sheet of black paper over the whole
of the white field and its black strip; rest the eyes; and
finally, when everything is in readiness, and the eyes again
fixed on the ball, swiftly draw away the black paper, keep-
ing the eyes motionless. The contrast colors are seen on
the instant, before any motions of the eyes that might intro-
duce successive contrast have been made.

Hering argues that this experiment is conclusive against
the psychological explanation of simultaneous contrast,
unless a separate unconscious judgment is to be made for
each eye; for that which is seen is a light violet field,
and the contrast color to that should be a greenish yellow,
and both images of the strip should be alike, whereas,
actually, the images appear in different colors, neither of
which is the color required.

Hering, J.

157. Induction of a Like Color. An effect the reverse
of the ordinary contrast effects sometimes appears, the in-
ducing color reappearing in the induced field.

a. Place close side by side a large piece of black paper
and an equal sized piece of white. Make a dot as a fixation
point at the middle of their line of junction, and stare
fixedly at it for half a minute. After a few seconds the
white will appear decidedly darker and the black decidedly
lighter, the effect becoming more marked as fixation is
continued. See also Ex. 122.

b. A darkening or brightening of a colored ground is often
to be observed when a figure in black or white is placed

Digitized by



upon it. This is a method of obtaining shades and tints
often used in polychromatic decoration. Observe the effect
in Fig. 4 of Plate I. The same may be observed occasion-
ally in plaid fabrics, and is shown very satisfactorily in
kindergarten mats woven in checker-board pattern of col-
ored and gray papers. If a set of graded grays is used so
that the strips may range evenly from a black at one side
to a white at the other, the corresponding shading of the
colored paper is striking.

On a, Helmholtz, A, 554 ff., Fr. 527 ff. (401 ff.); Hering, A, 36 ff.
On 6, von Bezold, #, 182-183 and Plate V. For what is perhaps a
related phenomenon, see Briicke, 424 ff. ; Helmholtz, A, 549, Fr.
520 (396); Aubert, A, 549 f.

158. Influence of Experience in Visual Perception. While
in the previous experiments a physiological explanation
seems sufficient for the facts, psychical action is not ex-
cluded, even by Hering, from a considerable share in sense
perception. In the following experiments experience co-
operates in the result.

a. Place upon the color-mixer a short-pointed star of
white cardboard, or even a square; when in sufficiently
rapid rotation, it appears as a white central circle sur-
rounded by a more or less transparent ring. While in this
condition bring behind it a broad strip of black .cardboard
of somewhat greater length than the diameter of the star
from point to point. As the edge of the card advances, it
can be seen not only behind the transparent ring, but, appar-
ently, also behind the opaque central circle, and the portions
of the latter in front of the black card seem darkened by
its presence. The illusion holds, though with a lightening
instead of a darkening effect, when a white card is moved
behind a black star. The illusion fails by degrees if the
card is kept motionless, but may be observed to a certain
extent when the star is at rest, or even on a square of card-

Digitized by



board held in the hand while another is moved to and fro
behind it. In all cases the latter card should often be
wholly withdrawn, so that its edge can be clearly seen,.

~ b. Cover a piece of black cardboard smoothly with tissue
paper, and notice that it seems at first blacker (because its
color is well known) than it afterwards proves to be on com-
parison with other grays.

- c. In mixing colors by reflection (Ex. 150 a), notice the
tendency to see one color through the other, instead of see-
ing the mixture of the two. This tendency may be so
strong at first as to interfere, to a certain extent, with the
success of the experiment. See also Ex. 164.

Helmholtz, A, 312, 323 f., Fr. 360 (273); Kirschmann, E. On the
difficulty of judging small differences in the color of surfaces that
present other small unlikenesses, see Hering, E.

Some Phenomena of Kotating Disks.
-K 159. The Mtinsterberg-Jastrow Phenomenon, a. Set a
black and white disk, e.g., that used in Ex. 145 a, in rapid
enough rotation to give a uniform gray ; pass rapidly before
it a thin wooden rod or thick wire, and notice the multitude
of shadowy images of the rod that appear on the disk. The
number of images is greatest in the portion of the disk
having the most frequent interchange of black and white.
b. Replace the disk by one carrying two or more colors.
Notice the repetition of the phenomenon, and that the
colors of the images are the colors (otherwise completely
blended) which the disk actually carries. The explanation
of the phenomenon is not altogether clear, biTt the sudden
changes of the background against which the rod is seen
seem to have an effect not unlike that of a stroboscopic disk
or of intermittent illumination, and thus show the rod at
rest in its successive positions.


Digitized by



160. Retinal Oscillation. Prepare a disk of black card-
board 25-30 cm. in diameter, and paste upon it a sector of
white of 90° extent. Put the disk in slow rotation (one turn
a second), fixate the middle of the disk, and notice that the
retreating edge of the black is always followed by a narrow
shadowy sector in the white. Under favorable conditions
more than one may be seen. The retina on first being stimu-
lated with white, apparently reacts in the direction of black
(see Ex. 125), then swings again toward white, and so on.

Charpentier, B.

• 161. Perception of Flicker with Different Parts of the
Retina. Place upon the color-mixer a black and white disk
in which the sectors are complete from centre to circumfer-
ence ; those used in Ex. 145 will not answer here. Rotate
the disk at such a rate as to give a lively flicker, fixate its
centre and slowly increase the rate. With care a point will
be found where the sectors are blended for the central parts
of the retina, but still flicker for the periphery. Try also
looking at one edge of the disk while giving attention to the
centre or opposite edge. This is in accord with the general
principle that peripheral after-images are of shorter duration
than those of the retinal centre. Too bright illumination
should be avoided, for with intense light the difference be-
tween the centre and periphery is less, or even quite reversed.
Bellarminow. On rotating disks and their phenomena in general,
see Helmholtz, A, 480-501, Fr. 445-471 (337-357).

Binocular Phenomena of Light and Color. 1
- 162. In general the two eyes co-operate to bring about
a single visual result, but the union of the impressions upon
the two retinae is influenced by a number of circumstances.

1 The experiments that follow can all be made with the stereoscope, but prac-
tice will enable the experimenter to combine the diagrams with free eyes, either
by crossing the lines of sight (fixating a point nearer than the diagram), or by
making them parallel or nearly so (fixating a point beyond the diagram). This

Online LibraryEdmund C. (Edmund Clark) SanfordA course in experimental psychology → online text (page 13 of 33)