THE COLORS O F COLLOIDS. VI BY WILDER D. BANCROFT
Blue Eyes Tyndalll pointed out that the blue color of eyes was undoubtedly the blue of turbid media, but this seems not to have been understood by the physiologists. Roberts2 says : “The iris, on which the color of the eye depends, is a thin membranous structure composed of unstriped muscular fibers, nerves, and blood-vessels, held together by a delicate network of fibrous tissue. On the inner surface of this membrane there is a layer of dark purple pigment called the weu (from its resemblance to the color of a ripe grape), and in brown eyes there is an additional layer of yellow (and perhaps brownred) pigment on its outer surface also, and in some instances there is a deposit of pigment among the fibrous structures. I n the albino, where the pigment is entirely absent from both surfaces of the iris, the bright red blood is seen through the semi-transparent fibrous tissues of a pink color; and in blue eyes, where the outer layer of pigment is wanting, the various shades are due to the dark inner layer of pigment-the uveashowing through fibrous structures of different densities or degrees of opacity. The eyes of new-born infants of both white and black races (and I believe the new-born young of all the lower animals) are dark blue, in consequence of the greater delicacy and transparency of the fibrous portion of the iris; and as these tissues become thickened by use, and by advancing age, the lighter shades of blue, and finally gray are produced; the gray, indeed, being chiefly due to the color of the fibrous tissues themselves. I n gray eyes, moreover, we see the first appearance of the superficial layer of yellow pigment in the form of isolated patches situated around the margin of the pupil, or in rays running across the iris. I n the 1
2.50
“Fragments of Science: The Sky.”
(1919). 2
Brit. Assoc. Reports, 50, 135 (1880).
See also Jour. Franklin Inst., 187,
*
The Colors of Colloids.
VI
357
hazel and brown eyes the .uvea and the fibrous tissues are hidden by increasing deposits of yellow and brown pigment on the anterior surface of the iris, and black eyes result when this is very dense. It is very doubtful, however, whether the iris is ever so dark colored in the inhabitants of this country as to justify the term black being applied to it, and the popular use of the expression has reference to the widely dilated pupil common in persons with dark brown eyes. The nearest approach to a black eye among us is the dark blue or violet eye associated with black hair in some Irish adults; here the color is probably not due entirely, as in infants, to the greater transparency of the fibrous structures, but to interstitial deposit of black pigment, or to a layer situated on the anterior surf ace of the iris. ’ ’ This view-point has been adopted in the other papers which I have read. Thus Darbishirel says: “When we speak of the color of the human eye, we are speaking only of the color of a particular part of it, namely theiris, which is the only part of the eye the color of which varies to any great extent. There is, of course, an almost infinite variety in the color of the eye; but the various types of color can be grouped into two large classes according to whether there is brown pigment on the outer surface of the iris or not. When the whole of the surface, which is the one that we see when we look at the eye, is covered with brown pigment on the outer surface of the iris the eye is generally brown. The word duplex has been coined to denote those eyes in which there is some brown pigment in front of the iris; and simplex to denote those eyes in which there is none.2 Speaking generally, duplex eyes are brown, and simplex are blue. But as the two classes, duplex and simplex, include the whole range of human eye color it is evident that duplex does not simply mean brown and simplex blue, because they also include the green, the gray, and the hazel eyes. “Breeding and Mendelian Discovery,” 41 (1911). These terms were introduced by Mr. C. C. Hurst, who was the first to discover, investigate, and record the Mendelian inheritance of eye color in man.
358
Wilder D. Bancroft
“A duplex eye is, as we have said, any eye in which there is brown pigment in front of the iris. This brown pigment exists on the top of the fibrous tissue of the iris, which appears blue; so that if we could dissolve away the brown pigment from a dark brown eye the blue would appear underneath. There is every degree in the amount of this brown pigment. If there is very little indeed, i t usually exists as a thin ring round the pupil, all the rest of the iris being blue. This has been called a ring-duplex, and if there is a mere trace of the brown pigment, a duplex eye of this kind would doubtless often pass as a gray or even blue eye. If there is more of the brown pigment a hazel eye is very often the result. I n another form of duplex eye the brown pigment is distributed in patches over the blue ground color which can be seen between the patches; hazel eyes are also often the result of this arrangement. When there is a very thin uniform layer of brown pigment, the color of this (a sort of yellow ochre) blends with the blue of the background, which can be seen through, and a green eye is the result. And finally there are the self-colored duplex eyes in which the brown pigment is spread uniformly over, and completely concealing the blue background, giving the various intensities of the brown eye, according to its abundance. “A simplex eye is one in which there is no brown pigment on the anterior surface of the iris. All the clear blue and gray eyes are of this class. The color of simplex eyes is not due to the color of the fibrous tissue of the iris itself, which consists of muscle fibers, nerves, blood-vessels, and so forth, but to a layer of dark purple pigment behind the iris, called the uvea. In the new-born infant the fibrous tissue of the iris is very delicate and transparent, so that most of the color of the uvea can be seen through it : that is why the eye of the new-born infant is dark blue. A clear blue eye is due to the delicacy of the fibrous tissue of the iris, which permits the color of the uvea to shine through. A pale blue or gray eye is due to the coarseness or ’stringiness’ of the fibrous tissue, which prevents most of the color of the uvea from being seen.
The Colors of Colloids.
VI
359
Evidence that the color of very pale blue and of gray eyes is due to the color of the uvea which can filter through the fibrous tissue in front of it, and not to the color of the fibrous tissue itself, is afforded by the fact that in albinos, in whom the uvea is colorless, the iris appears pink, this color being given by the blood in the fibrous tissue. The various color of simplex eyes, namely, the different intensities of blue and gray eyes and the intermediates between these two colors, are therefore due to differences in the texture of the fibrous portion of the iris, which allow different amounts of the color of the purple uvea behind to filter through. A simplex eye, therefore, is one in which there is only one layer of pigment, the purple of the uvea ; a duplex eye is one in which there are two layers of pigment, the purple of the uvea behind the iris and a varying amount of brown pigment in front of it. “If the reader prefers the use of familiar terms, duplex may be spoken of as brown and simplex as blue, and in nine cases out of ten the application of these common words will be correct; but i t must be remembered that there are some eyes which would be described as blue in common parlance, but which really fall into the duplex class because they possess a trace of brown pigment.” Davenport1 quotes Roberts and then adds a paragraph of his own. “While in most races of the globe, brown pigment is heavily secreted in the iris, in northwestern Europe blue, gray, or yellow-blue eyes are found. It seems probable that, once upon a time, or perhaps a t many times, an individual was born without brown pigment in the iris. The offspring of such prospered and spread throughout northwestern Europe and migrated thence to America and Australia. This defect, lack of eye pigment, has had a wonderful history. By noting its distribution the migrations of peoples can be traced. Thus Gray (1907)has shown that, in Scotland, pure blue eyes are most abundant in the coal and iron districts. This is probably due to the Irish immigrants, it being well known that blue eyes are very common among the Irish. “Heredity in Relation to Eugenics,”
27 (1911).
360
Wilder D. Bavtcroft
I n the Spey valley of Scotland the density of pure blue eyes is high-probably owing to the Norse invasion at that point. So in our own country the pigmentation survey that will some day be made will show a high percentage of blue eyes where the Scandinavians and north Germans have settled, The general point of view seems to me to be entirely wrong and I hope to be in a position before long to prove this experimentally. Roberts believes that blue eyes are due to the fact that the uvea is purple, and he considers that the marked blue in the infant’s eye is due to the iris being more transparent and thus permitting the color of the uvea to show through. It seems to me much more probable that the blue is the color of turbid media as suggested by Tyndall, and that the eye of the infant is more blue than that of the adult because the suspended particles are finer and therefore give rise to a richer blue. The object of the uvea is to prevent the red color of the blood showing through, as happens with albinos, and to cut down reflections from the back of the eye. I believe it to be practically unimportant whether the uvea is purple, or black, or dark brown. The development of pigment on the front of the eye will of course modify the color, a brown pigment giving all gradations between brown eyes and black eyes. If the brown pigment is yellow in very thin layers, we get a green eye. There should be no difficulty in duplicating this on a large scale in beakers or globes painted with a dark coating on the back, filled with a suitable turbid medium, and tinted to the desired extent in front. This should make a wonderful set of lecture experiments. It is well known that most infants have blue eyes, which often change to other colors as the children grow up. This merely means that the pigment on the front of the iris usually develops quite a while after birth, just as do the teeth. The unpigmented eye is always blue and the change is therefore always from blue to some other color and not in the reverse direction. If the pigmentation is marked a t birth, the child does not have blue eyes. I’
The Colors of Colloids. VI
361
The general conclusions of this paper are as follows: I . In blue eyes there is no pigment on the front of the iris. The blue color is the color of turbid media and is richer, the finer the suspended particles. 3. When the uvea is lacking, the color of the blood shows through and we have albinism. 4. The function of the uvea is to prevent the red of the blood showing through and to cut down reflections from the back of the eye. This intensifies the blue, just as the colors of thin films are more intense when oil is spilled on an asphalt pavement. The blueness of the eye does not depend to any great extent on the color of the uvea, though this may play a part in certain special cases. 5 . All other colors are due to pigmentation on the front of the eye which either modifies or masks the blue of the turbid media. 6. It should be a simple matter to duplicate experimentally on a large scale all the eye colors and thereby prove whether Tyndall or Roberts was right. 2.
Cornel1 University
