examining double stars when it was too bright to properly view nebulae.39 Bright moonlit nights would not be ideal conditions under which to search for double stars with very dim companions. If a search for planets had been motivat-ing his observing program, it was no longer an objective by this time. Double stars and the range of stellar luminosities From his surviving correspondence, there is no indication that John Herschel was motivated in his double star research by the possibility of observing planets orbiting other stars. Of course, the lack of evidence is not in itself proof that such motivations were unimportant to Herschel. There may be letters in which he spoke explicitly on this topic that have been lost, or he may simply have never committed his views on the subject to writing. The nature of Herschel’s surviving correspondence as well may help explain such an absence. The fact that Herschel retained drafts of many of the letters he sent to his scientific peers and that these drafts were later transcribed indicates that Herschel viewed his correspondence as potentially public. The speculative nature of his ideas on observing planets in orbit around other stars may have meant he would not have included it in letters that would possibly be made public at a later date. How then should one understand Herschel’s published comments on the possibility of observing planets orbiting other stars? The claim outlined below is that rather than situating Herschel’s 1833 comment in the context of a possible pre-existing research program that motivated his work, it should be taken instead to illustrate his changing perceptions on the range possible in stellar luminosities and the problem posed to his classification of double stars by a certain type of object, that of a bright star with a very dim companion. As Herschel began his work on double stars, a primary concern was distinguish- ing between binary doubles and optical doubles. The most certain way to make this distinction was to compare observations over a period of time in order to determine whether the component stars moved. Assumptions regarding stellar magnitudes, however, were superim- posed on considerations of gravitational dynamics. John’s father operated under the assumption that one could define classes of celestial objects that contained objects of relatively uniform properties.40 He used this assump-tion to gauge stellar distances (assuming dimmer stars were more distant and brighter stars less distant). Though there was growing awareness that stars varied intrinsi-cally in brightness, there was during this period no clear idea of how wide a range of stellar luminosities was possible. It is against this uncertainty that Herschel’s 1833 com-ment regarding the possibility of observing planetary com-panions of stars must be understood. Binary stars would be positioned closely together in space and therefore relatively similar in brightness. (Even William noted that systems he
assumed were binary often contained stars of differing magnitudes, but this did not cause him to abandon the assumption that most stars were of relatively similar intrinsic brightness.41) Though John Herschel explicitly distanced himself from this assumption in his 1826 pub-lished paper on stellar parallax and again in his 1833 Treatise on Astronomy, where he acknowledged that the magnitude of stars may differ ‘in the proportion of many millions to one,’ his correspondence with Struve indicated that he still assumed certain bounds on the luminosities of stars with respect to each other.42 Indeed, even within the Treatise he was not consistent in his treatment of stellar luminosities, at times assuming a relative uniformity in his analysis of the band of the Milky Way.43