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The Helmholtz ophthalmoscope, invented by Hermann von Helmholtz in the 1850s, revolutionized the study of the eye by allowing doctors to visualize the internal structures of the eye. Allvar Gullstrand, a Swedish ophthalmologist and physicist, conducted extensive research using the ophthalmoscope and made significant contributions to our understanding of the optics of the eye, including the measurement of astigmatism and the development of new methods for correcting visual defects.