Kant reads Wright

Far from London in the East Prussian capital of Konigsberg, the philosopher Immanuel Kant, 13 years younger than Wright, read a review of Wright's Original Theory of 1750. The review appeared in a periodical, and summarized Wright's models without reproducing his engraved illustrations. Therein lies the root of a deep misunderstanding, because without Wright's illustrations to guide him, Kant formed an erroneous impression of the details of Wright's models.

Kant's imagination was fired by a statement in the summary comparing the system of stars to our solar system of planets orbiting the Sun. The summary did not properly emphasize that in Wright's view, the stars orbit their divine center — a spiritual space, devoid of matter — and do so at some distance from it. Kant immediately conceived of a disk system, similar in fact to the modern conception of our galaxy, in which the stars are spread throughout the system. In his cosmological dissertation Universal Natural History and Theory of the Heavens, printed anonymously in 1755, he wrote:

''Mr. Wright of Durham, whose treatise I have come to know from the Hamburg publication entitled the Freie Urteile, of 1751, first suggested ideas that led me to regard the fixed stars not as a swarm scattered without visible order, but as a system which has the greatest resemblance with that of the planets; so that just as the planets in their system are found very nearly in a common plane, the fixed stars are also related in their positions, as nearly as possible, to a certain plane which must be conceived as drawn through the whole heavens, and by their being very closely massed in it they present that streak of light which is called the Milky Way.''58

Because the Milky Way appears to encircle the sky, Kant added that ''our sun must be situated very near this great plane,'' or be part of it.

Having reasoned to his satisfaction that the stars form a disk-shaped system, and that a stellar disk surrounding us accounts perfectly for the appearance of the Milky Way, it was a short step for Kant to propose that similar stellar systems, which we would now call galaxies, dot the infinite space of creation. He had read descriptions of ''nebulous stars,'' and believed these were best explained as distant stellar agglomerations. These stellar systems might also be disk-shaped; indeed, some references in the astronomical literature seemed to provide support for his idea.

In particular, Kant had read of observations by the astronomer Pierre de Maupertuis. De Maupertuis had represented nebulous objects, Kant noted, as ''small luminous patches, only a little more brilliant than the dark background of the heavens; they are presented in all quarters [of the sky]; they present the figure of ellipses more or less open; and their light is much feebler than that of any object we can perceive in the heavens.''59

De Maupertuis's description of the shape of nebular stars, ranging from roughly circular to elongated or elliptical, seemed to fit what one would expect for distant stellar disks — not, as Wright would have it, spheres or rings — observed from a variety of viewpoints. If viewed edge-on, the disk of stars would appear highly elongated; if the disk happened to face the observer, it would appear round (see figure 3.8). Disks at intermediate angles, neither face-on nor edge-on, would appear elliptical. The feebleness of the light simply meant, to Kant, that the systems were at ''inconceivable'' distances.

''I easily persuaded myself that these [''nebulous''] stars can be nothing else than a mass of many fixed stars,'' Kant wrote in his preface.60 Without bothering to check them out for himself, the philosopher admitted them as evidence and laid out his case.

''I come now to that part of my theory which gives it its greatest charm, by the sublime ideas which it presents of the plan of creation,'' Kant declared in the first chapter. He asked his

Figure 3.8 A disk of stars, or galaxy, viewed face-on (left view) and edge-on (right view). The middle view is for an intermediate viewing angle. (Credit: Layne Lundstrom.)

reader to imagine the consequences if ''a system of fixed stars which are related in their positions to a common plane, as we have delineated the Milky Way to be, be so far removed from us that the individual stars of which it consists are no longer sensibly distinguishable even by the telescope.'' He concluded, ''if such a world of fixed stars is beheld at such an immense distance from the eye of the spectator situated outside of it, then this world will appear under a small angle as a patch of space whose figure will be circular if its plane is presented directly to the eye, and elliptical if it is seen from the side or obliquely. The feebleness of its light, its figure, and the apparent size of its diameter will clearly distinguish such a phenomenon when it is presented, from all the stars that are seen single.''61

So an early version of the concept of ''island universes'' hatched, a product of Wright's pioneering effort to explain the appearance of the Milky Way and Kant's bold synthesis of observation and theory. Kant's argument is not as strong as it might at first seem, in adducing de Maupertuis's data to support the concept of a multitude of stellar systems similar to our own. It turns out that only one of the ''nebulous stars'' described by the astronomer—the Andromeda Nebula — is, in fact, a galaxy, a stellar system comparable to our own; the other nebulous objects included a small number of globular clusters and the Orion nebula, which hardly anyone could describe as round or elliptical. Further examples of true ''island universes'' or galaxies besides our own weren't published until 1755. Nevertheless, both Wright's and Kant's insights, riddled as they were with misunderstandings, represented real progress in scrutinizing what lies beyond the solar system.

Kant, like Wright, had no doubt that his effort to explain the appearance of the Milky Way was new. Kant was extremely pleased with his ideas, but so mindful of their simplicity that he wondered why no one else had come up with them. He remarked that Wright did not sufficiently observe the potential applications and ramifications of his stellar system model, and took all astronomers to task for not even considering the problem posed by the Milky Way. In his characteristically long-winded style Kant wrote:

''Whoever turns his eye to the starry heavens on a clear night, will perceive that streak or band of light which on account of the multitude of stars that are accumulated there more than elsewhere, and by their getting perceptibly lost in the great distance, presents a uniform light which has been designated by the name Milky Way. It is astonishing that the observers of the heavens have not long since been moved by the character of this perceptibly distinctive zone in the heavens, to deduce from it special determinations regarding the position and distribution of the fixed stars.''62

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