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As regards the "aromatics" of chemistry, he accepts the theory of Zwaardemaker and others that certain groups of atoms, the osmophors," in the molecule have to do with odor, but points out that substances with different constitution as regards osmophors may smell alike. Three factors, he says, determine odor: (1) the presence of certain osmophors; (2) a kernel or radical for the molecule, to which the osmophors are attached (an welcher sie sitzen), which Henning calls the "osmogen" and which may consist of one or more atoms; and (3) the manner in which the omsophors are bound to the radical, that is, the positions they take in the benzol rings. In short, smell in the aromatics is largely a matter of molecular pattern. Henning finds patterns to correspond to each of his six classes of smells, and to the transitions between them. Tastes, also, in the case of organic substances are controlled by groups of atoms, though no one taste depends upon any one group. The group NO2, for example, is characteristic of bitter substances, NH of sweet, and COOH of sour. With regard to inorganic salts Henning points out that since each has a different taste, quality cannot depend simply on the presence of anions or kations but must depend upon some relation between the two-perhaps, as Herlitzka suggested, on a difference in solution pressure. Incidentally, it may be said that Henning agrees with Zwaardemaker (7) and disagrees with Teudt in believing that electrical effects have little to do with smell.

Among other matters with regard to taste and smell which Henning treats with full reference to the literature and with illuminating comments are stimulus thresholds, reaction times, imagery, illusions and hallucinations, feeling-tone, the physiology of the organs, and smell and taste in animals. His study of smell in ants, which forms the first appendix to his monograph, is a most noteworthy contribution to animal psychology. No space remains for a discussion of these topics here. For a clear, comprehensive and interesting survey of the psychology of taste the reader is recommended to Hollingworth and Poffenberger's little book (8). Unfortunately this book went to press just before Henning's work was known in this country. Whether one agrees with Henning or not, one must admit that any book on taste or smell which fails to reckon with him is somewhat out of date. Since, however, Henning's opinions on taste are not very revolutionary except with regard to the list and relations of simple tastes, Hollingworth and Poffenberger may congratulate

themselves that it was a book on taste and not one on smell that they published in 1917.

Incidentally it should be noted that Hennschen (9), on the basis of a series of examinations of the brains of persons with taste defects, has been led to believe that the cortical centre for taste is not in the horn of Ammon nor in the hippocampal gyrus. He is, however, not very certain even of this negative conclusion.

Finally a word should be said about Zwaardemaker's review of Henning's monograph. Zwaardemaker (6), in view of Henning's scathing and repeated criticisms, has treated the younger writer most magnanimously. He speaks cordially of Henning's psychological work and with appreciation of his wide knowledge of the literature in his field. The almost too marvellous correspondence between Henning's psychology and chemistry of smell, however, is pointed out. This chemistry Zwaardemaker declines for the time being to criticize. His severest criticism amounts to saying that Henning as a psychologist ought not so often to step out of his own field in order to criticize the work of physiologists, to whose technique he is not in a position to do justice.

REFERENCES

I. HENNING, H. Der Geruch. Leipzig, Barth, 1916. Pp. 533. The contents of this book, except for the indices, appeared in the Zeits. f. Psychol. for 1915 and 1916.

2. HENNING, H. Die Physiologie und Psychologie des Geruchs. Ergebnisse d. Physiol. of Asher and Spiro, 1919, 17, 572-627.

3. HENNING, H. Die Physiologie und Psychologie des Geschmacks. Ergebnisse d. Physiol., 1921, 19, 1-78.

4. GAMBLE, E. A. McC. Der Geruch by Hans Henning. Amer. J. of Psychol., 1921, 32, 290-295.

5. HENNING, H. Experimentelle Untersuchungen von Denkpsychologie. I. Die associative Mischwirkung, das Vorstellen von noch nie Wahrgenommenen und ihre Grenzen. Zeits. f. Psychol., 1919, 81, 1-96. 6. ZWAARDEMAKER, H., Ausführliche Rezension von Henning's Monographie. Arch. f. Ohren-, Nasen- und Kehlkopfheilkunde, 1916, 101, 245-249. 7. ZWAARDEMAKER, H., H. R. KNOOPS and M. W. VAN DER BIJL. The Electrical Phenomena in Cloudlike Condensed Odorous Water Vapour. Proc. Kon. Acad. van Wetensch te Amsterdam, 1916, 19, 44-49.

8. HOLLINGWORTH, H. L. and A. T. POFFENBERGER. The Sense of Taste. Moffat, Tard. and Co., New York, 1917, 200 pp.

9. HENNSCHEN, E. S. Ueber die Geruchs- und Geschmackszentren. Monatens. f. Psychiat. und Neurol, 1919, 45, 121-164.

THE FLIGHT OF COLORS IN THE AFTER IMAGE OF A BRIGHT LIGHT

BY WILLIAM BERRY
University of Chicago

It is well known that the after image of a very bright light presents the appearance of a series of color changes which persist for a longer or shorter time after the stimulus has been removed. The phenomena have received the name of Farbiges Abklingen der Nachbilder (Helmholtz). They have been more or less superficially noted from the time of Aristotle down to the modern period of Physiological Optics. Much of the literature of the subject is in German and French; there being comparatively few original articles in English. Almost all the texts in psychology and physiology refer to the phenomena, usually in most general terms, in the sections devoted to the visual sensations.

The purpose of this report is to review the literature of the subject and to present data drawn from this literature, which in many respects are very conflicting. It may seem to be a rather hopeless task to add anything new to the solution of a problem which interested a Fechner and a Seguin, or bring order out of such conflicting data. A review of the literature, however, suggests, in part at least, the reasons for the disparity between the results obtained. There are several problems which emerge from the review which seem to demand further experimental work.1

According to Aubert (6) the first person to make observations on the chromatic Abklingen der Nachbilder was Josephus Bonacursius. Aubert's reference is to Kircher's Ars magna lucis et umbrae, 1671. In point of fact there are several references to be found in the literature prior to this date. Aristotle reveals a very carefully made observation in the passage in De Insomniis (3) which has been translated as follows: Again, if after having looked at the sun or some other bright object, we close the eyes, then, if we watch carefully, it appears in a right line with the direction of vision, at first with its own color, then it changes to crimson, next to purple, until it becomes black and disappears" (4). Following Aristotle almost everyone who wrote on the subject of optics made some sort of generalization on the phenomena and the features of the color transformation in the image. Almost all the observers used the sun as the primary stimulus and then with closed and covered eyes, or closed eyes, or with open eyes observed the shift of colors in the visual field. Plateau (49) has collected the references in the literature upon this and related topics in the field of subjective vision from the time of the Greeks down to the end of the 18th century. In connection with the subject of the color changes in the image, Plateau prefaces his annotated bibliography with some pertinent remarks on the danger to the eyesight in the investigation of the after image of blinding lights such as were used. "The experiments which are the object of this section are dangerous. They are of the sort which have developed in me the germ of an affection which has completely deprived me of vision. I cannot too strongly advise physicists and physiologists to abstain from the same experiments, which present but a slight importance compared with the ills which may arise."

1 The writer has carried on a prolonged series of observations with a number of subjects on the flight of colors in the after image of a bright light. The results of these observations will be published later.

Certainly the rash gazing at the strong light of the sun through a window or even directly in the broad light of day, which seems to have been the method of procedure of all these observers up till quite recent times could not be other than highly imprudent and fraught with evil consequences to the eyes of the unfortunate observer. Helmholtz and Fechner gave specific utterance to a similar warning. Unfortunately in the case of Fechner the warning was not heeded until it was almost too late to prevent unhappy consequences to his eyesight.

As this bibliography of the literature compiled by Plateau is not easily accessible to the average reader, I have availed myself of his work in the preparation of the following summary of the literature from the earliest period. For the most part the references are more or less casual statements without any attempt to elaborate the description or give specific details. This is true of many of the original sources from which Plateau obtained his material. I have, therefore, omitted from this account all but the mere statement of the sort of stimulus which was used in every case and the sequence of colors as it was reported in each case.

Summary of the reports made on the sequence of colors in the after image of a bright light, from the Greek period to the end of the 18th century.

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Aepinus3

Sun

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Scherffer

Mongez

De Godart

Venturi

Candle flame Natural color, with a black border, changes to bright red, green, then a mixture of green and dark blue, then the image appears black. Dark blue, green, reddish orange, purple.

White cloud

White paper
in sun's rays
Sun

White paper
in the sun-
light

Sun, candle
flame, white
paper in sun

Dark blue, green, orange, purple.

Red, orange, yellow, green, blue, indigo, violet.

Pale yellow, golden yellow, yellowish green, green, violet, blue, purple, reddish brown.

White, yellow, red, indigo, blue,

green.

1 Plateau briefly notes the observations of Bonacursius in his review. As the volume of Kircher's Ars magna lucis et umbrae could not be found by the writer, the following account has been taken from Aubert (5): "Among other observations Josephus Bonacursius mentions this In any place which is closed as completely as possible so that no light whatever may enter, leave an opening covered with paper on which you draw any figure, or preferably, silhouettes of things. Also let the opening be facing the sun so that it may be illuminated by the sun. Having arranged this, fix the eyes on the paper opening and gaze steadily for a considerable time till the retina of the eye has taken in the image completely. Then, having closed the window fix the eyes on a white paper, and lo, wonder to relate on the paper you will see as it were the rising dawn, yellow being generated first, then red, next purple and lastly all the colors of the rainbow and finally

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