In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers. Possibly as numerous as conventional galaxies, these galaxies have the same general shape and even the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies' mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars—that is, they evolve much more slowly.

    These galaxies may constitute an answer to the longstanding puzzle of the missing baryonic mass in the universe. Baryons—subatomic particles that are generally protons or neutrons—are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy, suggests that there are far more baryons in the universe than estimates based on galactic luminosity indicate. Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.

    Linda Kerber argued in the mid-1980's that after the American Revolution (1775-1783), an ideology of "republican motherhood" resulted in a surge of educational opportunities for women in the United States. Kerber maintained that the leaders of the new nation wanted women to be educated in order to raise politically virtuous sons. A virtuous citizenry was considered essential to the success of the country's republican form of government; virtue was to be instilled not only by churches and schools, but by families, where the mother's role was crucial. Thus, according to Kerber, motherhood became pivotal to the fate of the republic, providing justification for an unprecedented attention to female education.

    Introduction of the republican motherhood thesis dramatically changed historiography. Prior to Kerber's work, educational historians barely mentioned women and girls; Thomas Woody's 1929 work is the notable exception. Examining newspaper advertisements for academies, Woody found that educational opportunities increased for both girls and boys around 1750. Pointing to "An Essay on Woman" (1753) as reflecting a shift in view, Woody also claimed that practical education for females had many advocates before the Revolution. Woody's evidence challenges the notion that the Revolution changed attitudes regarding female education, although it may have accelerated earlier trends. Historians' reliance on Kerber's "republican motherhood" thesis may have obscured the presence of these trends, making it difficult to determine to what extent the Revolution really changed women's lives.

    In American Genesis, which covers the century of technological innovation in the United States beginning in 1876,Thomas Hughes assigns special prominence to Thomas Edison as archetype of the independent nineteenth-century inventor. However, Hughes virtually ignores Edison's famous contemporary and notorious adversary in the field of electric light and power,George Westinghouse. This comparative neglect of Westinghouse is consistent with other recent historians' works, although it marks an intriguing departure from the prevailing view during the inventors' lifetimes (and for decades afterward) of Edison and Westinghouse as the two "pioneer innovators" of the electrical industry.

    My recent reevaluation of Westinghouse, facilitated by materials found in railroad archives, suggests that while Westinghouse and Edison shared important traits as inventors, they differed markedly in their approach to the business aspects of innovation. For Edison as an inventor, novelty was always paramount: the overriding goal of the business of innovation was simply to generate funding for new inventions. Edison therefore undertook just enough sales, product development, and manufacturing to accomplish this. Westinghouse, however, shared the attitudes of the railroads and other industries for whom he developed innovations: product development, standardization, system, and order were top priorities. Westinghouse thus better exemplifies the systematic approach to technological development that would become a hallmark of modern corporate research and development.