 As discussed above, in the seventeenth century both Hooke and van Leeuwenhoek pioneered the use of light microscopes to observe cells. Subsequent investigators often designed their own microscopes. The variations in these designs contributed to variability in the resulting images. (Before photography, microscopists drew what they saw using the microscope, introducing another source of variation.) The variability of the images different researchers produced was one factor that led biologists in the eighteenth and nineteenth centuries to examine more carefully the processes through which microscopes generate images (Schickore 2001, 2007). In his theoretical studies of lenses, Newton (1704) characterized two types of aberrations created by lenses: spherical aberrations, resulting from light rays coming into focus at different points, and chromatic aberrations, resulting from light of different wavelengths being refracted at different angles. Schickore (2007) describes the many efforts by microscope developers to correct for these aberrations and the creation of test objects for evaluating the reliability of particular microscopes. During the same time enthusiasts were advancing many claims about what they saw, some of which were shown later to be artifacts. For example, both Milne-Edwards (1823) and Dutrochet (1824) reported round structures of a constant size similar to that reported for cells, which they termed globules. (See Schickore 2009 for a detailed examination of the reports of globules and an argument that it was the variability in these reports that contributed to the growing sense that something was amiss in the practices of the microscopists.) Globules were, however, soon shown to be the products of spherical aberrations. In the early nineteenth century several lens makers developed strategies for eliminating spherical aberrations and greatly limiting chromatic aberration (chromatic aberrations were not fully eliminated until the introduction of the apochromatic lenses in mid-century). As a result, the observations by Schleiden, Schwann, and others discussed above were largely free of these distortions.