Though Caroline had a personal interest in defending her brother’s theories, Herschel’s correspondence with other astronomers – specifically the generation that had been active during his father’s life – showed his desire to establish that his own observations added to the credibility of his father’s original theory. For example, in 1825 Herschel wrote to the French astronomer Pierre Simon Laplace (1749–1827) telling Laplace that his work with South on double stars included ‘many interesting verifica-tions of those views which I remember you once characterized. . . as ‘tres Philosophiques en tres vraise.’’28 Likewise in a letter to Joseph Fourier (1768–1830) this same year, he noted that his own work, along with that of South and Struve, had verified his father’s discovery of ‘the very important fact’ that many stars form binary systems.29 From early on, Herschel saw his work on double stars integrally linked with that of his father’s and presented it in terms of confirmation and not discovery. Though by the mid-1820s Herschel felt confident that his work had confirmed his father’s theory, he continued to indicate that collecting further proof of binary stars remained an important consideration in his research. In May 1831 he summarized the state of double star astron-omy in a letter to his aunt, noting that ‘the Double Stars flourish exceedingly’ and that there were now four such objects for which orbital periods had been determined. In this letter, Herschel emphasized even more the signifi-cance of these orbits as a vindication of his father’s work. He wrote that they represented ‘a new department of astronomy of which before my Father’s time not a soul had the least suspicion, and whose reality everybody doubt-ed. . . but which is now placed beyond a possibility of doubt.’ Indeed, Herschel went on to compare this discovery to that of the planet Uranus, arguing that, ‘The discovery of the Georgium Sidus was but a trifle compared to this, which I look upon as one of the greatest ever made by man.’30 What of the elder Herschel’s original motivation for surveying double stars, the idea of using optical doubles to determine parallax? John Herschel’s correspondence indicates that though this was an important consideration, it was secondary to his interest in the orbital determina-tion of binaries. As early as 1825 Herschel wrote to his aunt on the topic of parallax, though he was quick to deny any claims of having measured it. ‘Do not suppose that I pretend to have discovered parallax,’ he wrote, ‘but if it exists to a sensible amount, I think it cannot long remain undiscovered if anybody can be found to put into execution the method I am about to propose, and I hope it will be taken up by astronomers in general.’31 The paper to which he referred was one that would be published the following year in which he outlined a new method for using optical doubles to measure parallax. Previous methods for determining parallax with optical doubles focused on measuring the change in angular distance between the two stars. Obtaining this measurement, Herschel main-tained, was beyond current instrument precision, but measuring change in the relative angular position of the stars was not. According to Herschel no one had previously proposed this method of determining parallax, and in the paper he set out a detailed mathematical explanation and observing program.32 From this paper it is clear that Herschel was not inter-ested in being the first astronomer to determine the par-allax of a star, as significant a discovery as that would have been. Far from guarding his method until he could put it into practice and claim priority, he laid out exactly how one would go about making the discovery. Not only did he recommend the instruments to use and provide a list of stars that would be good candidates for observation (including the best times of the year to view them), he also provided an example of a working-through of his mathe-matical method for an individual star so those ‘not con-versant with algebraic symbols’ could extend his list of target stars if they desired.33 Yet despite having a theore-tically workable method, Herschel expressed no interest in actually conducting the measurements needed to deter-mine stellar parallax. In this paper Herschel also outlined his views regarding the luminosities of stars, an issue important in interpret-ing his later suggestion regarding observing planets in orbit around stars. Near the beginning of the paper he wrote that when selecting double stars to observe for parallax, ‘we ought by no means to confine ourselves, by assuming it as a universal law that the brightest stars are the nearest to us.’ Since there is no certain method of measuring the absolute brightness of stars, he continued, ‘it seems improbable that the real magnitude and bright-ness of the stars should be confined within narrow limits.’ Because of this, assumptions regarding the distances to stars could not be based on