G. B. Airy's model eye.

FOLLOWING Kepler's theoretical account of the formation of the retinal images, the first practical investigations appear to be those of the Jesuit mathematician, Christopher Scheiner (1575 1650).1 Scheiner was probably the first to construct a model eye, complete with cornea, crystalline lens, spherical glass retina, and two chambers filled with water.2 Shortly after Scheiner's experiments, many model eyes subsequently made their appearance and were constructed by anatomists, philosophers, and opticians, each model serving to demonstrate a particular optical principle or philosophical concept.3 By the end of the eighteenth century, the model eye was mainly employed to demonstrate the principles of vision generally, and the optical conditions and corrections of myopia, presbyopia, and aphakia. In 1828, George (later Sir George) Biddell Airy (1801-1892), having been Lucasian Professor of Mathematics and Plumian Professor of Astronomy at Cambridge, was appointed, at the age of 27, Director of the new Cambridge University Observatory. The letter which follows, describing the construction of a model eye, was written during his first year at the Observatory, in answer to an enquiry from Dr. Dionysius Lardner (1793-1859) concerning the model. From the general tone of Airy's letter to Lardner, it would appear that Airy had constructed the eye a considerable time before 1828. Lardner, who in 1827 had been elected to the chair of natural philosophy and astronomy, at the newly founded London University (now University College), may have requested information about the model eye for demonstration purposes. The letter (fig. 1), which is now in the Wellcome Historical Medical Library, has not previously been published, and reads as follows: