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The absence of the Purkinje phenomenon in the Fovea
NOTES
FROM
THE
NELA
RESEARCH
LABORATORY.*
T H E A B S E N C E OF T H E P U R K I N J E P H E N O M E N O N THE FOVEA.
IN
By Leonard T. Troland.
CAREFUL experiments by many observers have indicated that the Purkinjc phenomenon cannot be obtained if the compared visual stimuli (e.g.,a red and a blue light) are confined to the central rod-free area of the retina. Recently, I however, Hering has claimed that a very fugitive Purkinje effect can bc observed during the first moments of stimulation of the fovca of a dark-adapted eye. Since the conditions under which Hering's experiments wcrc performed do not appear to have been favorable to accurate limitation of the stimulation of the fovea, and on account of the great theoretical importance of the question, it seemed worth while to repeat the work, using an improved method. Hcring employed a field 2.o8 degrees in diameter, one half of which was filled with spectral blue light and the other with red. The experiment was carried out by extinguishing the blue, then suddenly fixating the red and at the same instant turning on the blue. Three defects can be pointed out in this method: (I) The field size is too great; (2) local minuthesis (" fatigue") of the red sensation can occur before the blue stimulus becomes effective; (3) steady fixation would be very difficultto obtain at thc criticalinstant, by this method. In the writer's experiments a field of one degree diameter was employed. This was crossed by a central red belt, 0.34 degree wide, made up of light of wave-lengths, 654-687 m*. The two equal polar segments were filled with blue light of wave-lengths, 469-481 m*. The whole was viewed against a dark background, through an artifical pupil of 1.36 ram. diameter. In the centre of the field was placed a luminous fixation point of o.o6o degree diameter, the brightness of which was just sufficient to make it visible to the dark-adapted eye. The subject was given five minutes' bright adaptation out-ofdoors, in the late morning or early afternoon, with a clear sky and May sun. He was then immediately .required to adjust the
----CComm-un~cated; ; the ISirecctor.-. . . . . . . . . . . . . . . . . . . . . . ' Arch. f. Ophlhalmol.,
1915, 90, I-I3. III
1 12
NELA RESEARCH LABORATORY NOTES,
[J. F. I.
relative intensities of the red and blue components of the test field so that either ( I ) the red was slightly but distinctly brighter than the blue, or (2) the two were of equal brightness. The intensity of the blue during this setting was 50 photons (see footnote to next article). The subject was now given 3° minutes' complete dark adaptation, after which he was required to fixate carefully the luminous point. Immediately upon establishment of fixation the original field was suddenly exposed to view, with the first selected relative intensities accurately maintained, but with a reduction of the absolute intensity of both components to one thirty-second of the value at which the bright-adaptation setting was made. This reduction was accomplished by means of a sector disk. Ten successive brief exposures of the field were made in this way, with a minute's rest between, and the subject was asked to judge whether, at the first instant of visibility of the field, the blue was brighter than the red (the Purkinje change ). In four separate experiments of this sort the writer found it impossible to detect the slightest trace of a Purkinje effect when fixation was accurately maintained. Peripheral fixation gave a vivid enhancement of the blue. Observations by two inexperienced subjects under the same conditions yielded conflicting results, which are to be attributed to their inability to fixate accurately. The experiment was also tried by the writer with an initial intensity of 31 photons, and eo minutes' bright adaptation, with similarly negative results. June i8, I916, THE
HETEROCHROMATIC BRIGHTNESS THRESHOLD.
DISCRIMINATION
B y Leonard T. Troland.
THE measurements described below were made to study some of the principles underlying the "direct comparison" method of heterochromatic photometry. Although it is always difficult to make a positive judgment that two stimuli of different color are of equal brightness, a ratio of intensities can easily be established for which a perfectly secure judgment of inequality of brightness can be delivered. Consequently, for any pair of colors it is possible to determine the briahtness discrimination threshold, and this should be a function
FROM
THE
NELA
RESEARCH
LABORATORY.*
T H E A B S E N C E OF T H E P U R K I N J E P H E N O M E N O N THE FOVEA.
IN
By Leonard T. Troland.
CAREFUL experiments by many observers have indicated that the Purkinjc phenomenon cannot be obtained if the compared visual stimuli (e.g.,a red and a blue light) are confined to the central rod-free area of the retina. Recently, I however, Hering has claimed that a very fugitive Purkinje effect can bc observed during the first moments of stimulation of the fovca of a dark-adapted eye. Since the conditions under which Hering's experiments wcrc performed do not appear to have been favorable to accurate limitation of the stimulation of the fovea, and on account of the great theoretical importance of the question, it seemed worth while to repeat the work, using an improved method. Hcring employed a field 2.o8 degrees in diameter, one half of which was filled with spectral blue light and the other with red. The experiment was carried out by extinguishing the blue, then suddenly fixating the red and at the same instant turning on the blue. Three defects can be pointed out in this method: (I) The field size is too great; (2) local minuthesis (" fatigue") of the red sensation can occur before the blue stimulus becomes effective; (3) steady fixation would be very difficultto obtain at thc criticalinstant, by this method. In the writer's experiments a field of one degree diameter was employed. This was crossed by a central red belt, 0.34 degree wide, made up of light of wave-lengths, 654-687 m*. The two equal polar segments were filled with blue light of wave-lengths, 469-481 m*. The whole was viewed against a dark background, through an artifical pupil of 1.36 ram. diameter. In the centre of the field was placed a luminous fixation point of o.o6o degree diameter, the brightness of which was just sufficient to make it visible to the dark-adapted eye. The subject was given five minutes' bright adaptation out-ofdoors, in the late morning or early afternoon, with a clear sky and May sun. He was then immediately .required to adjust the
----CComm-un~cated; ; the ISirecctor.-. . . . . . . . . . . . . . . . . . . . . . ' Arch. f. Ophlhalmol.,
1915, 90, I-I3. III
1 12
NELA RESEARCH LABORATORY NOTES,
[J. F. I.
relative intensities of the red and blue components of the test field so that either ( I ) the red was slightly but distinctly brighter than the blue, or (2) the two were of equal brightness. The intensity of the blue during this setting was 50 photons (see footnote to next article). The subject was now given 3° minutes' complete dark adaptation, after which he was required to fixate carefully the luminous point. Immediately upon establishment of fixation the original field was suddenly exposed to view, with the first selected relative intensities accurately maintained, but with a reduction of the absolute intensity of both components to one thirty-second of the value at which the bright-adaptation setting was made. This reduction was accomplished by means of a sector disk. Ten successive brief exposures of the field were made in this way, with a minute's rest between, and the subject was asked to judge whether, at the first instant of visibility of the field, the blue was brighter than the red (the Purkinje change ). In four separate experiments of this sort the writer found it impossible to detect the slightest trace of a Purkinje effect when fixation was accurately maintained. Peripheral fixation gave a vivid enhancement of the blue. Observations by two inexperienced subjects under the same conditions yielded conflicting results, which are to be attributed to their inability to fixate accurately. The experiment was also tried by the writer with an initial intensity of 31 photons, and eo minutes' bright adaptation, with similarly negative results. June i8, I916, THE
HETEROCHROMATIC BRIGHTNESS THRESHOLD.
DISCRIMINATION
B y Leonard T. Troland.
THE measurements described below were made to study some of the principles underlying the "direct comparison" method of heterochromatic photometry. Although it is always difficult to make a positive judgment that two stimuli of different color are of equal brightness, a ratio of intensities can easily be established for which a perfectly secure judgment of inequality of brightness can be delivered. Consequently, for any pair of colors it is possible to determine the briahtness discrimination threshold, and this should be a function