PubMed · 13043614
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A SCHLOSSMAN. 1953. Trifocals.. https://pubmed.ncbi.nlm.nih.gov/13043614/
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We discuss many aspects of a reproduction of a formal painting from the XVIth century. It is a portrait of Viceroy of New Spain Luis de Velasco, El Joven or Hijo (son-see text), found at the Museum of the Pecos National Monument (near Pecos and Santa Fe, New Mexico). A formal portrait of each Viceroy of New Spain was created, and this one was painted during de Velasco's first term as Viceroy (1590 to 1595 AD). In this depiction, he is seen wearing spectacles. To our knowledge, this is the first known reference suggesting the introduction of this form of visual correction in the Spanish Colonies in the Americas and in the future United States. There are three known portraits of Luis de Velasco (son): One painting, which included his father and significant others, hangs in the Colegiata (a small cathedral) de San Luis near Valladolid, Spain. In this portrayal, he was shown as a young man without a spectacle correction. And there are two viceregal portraits (Term 1, 1590 to 1595; Term 2, 1607 to 1611) in which he wears the same spectacle correction (these are located in Mexico City).
During off-axis viewing through an anisometropic spectacle correction, the two eyes must diverge to fuse. This can cause great discomfort, especially in the vertical meridian. To consider a correction for the problem, one must know the amount of forced divergence induced by the spectacles. Because the eyes select ray entry points that do not coincide with the object point projections on the lens planes, the exact directions of gaze are difficult to determine with conventional ray-tracing methods. The methods previously developed for determining the visual directions of gaze through anisometropic spectacle corrections are limited to hypothetical thin lenses and require complicated trigonometric constructs that are not suitable for clinical work. A recently developed method for solving the problem applies the dynamic spectacle magnification to find the visual direction of an image of a given object point. The method, referred to as the theory of object and image eccentricities, is based on projections of object and image from the rotation center onto a common plane, such as the back vertex plane. The theory can be applied to many situations in ophthalmic optics previously difficult to analyze. It does not require ray tracing, and because it considers base curves and thickness, it is more accurate than previous methods. A major advantage is that it considers the two eyes as an integrated system rather than separately and in isolation from each other.
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