Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures (1914)

672 DEMONSTRATING REFRACTIVE EYE DEFECTS [CH. XV diopter lens gives a smaller image than the 4 diopter lens* (see also fig. 389 B. C.). B Loft Eye Right Eye Left Eye Right Eye FIG. 402. APPEARANCE OF THE SCREEN IMAGE WITH UNLIKE REFRACTION IN THE Two EYES AND WITH LIKE REFRACTION. A Image of a 3 diopter left eye and a 4 diopter right eye; the 4 diopter eye is in focus. B Image when a i diopter convex spectacle is added to the 3 diopter left eye, making it like the right eye. § 932a. In the experiment showing anisometropia each half of the double lens projects both images, but when the light is properly centered and in the correct position to give the large illumination on the lens (fig. 400), each half lens projects a much more brilliant image of its own side, hence the fainter image of the opposite side is overwhelmed and overlaid so that only one image shows on each side. If the light is not in a good position, both images show and that spoils the effect. This demonstration with two half lenses was fully successful only when a right-angle, direct current arc lamp was used as a source of light. By using a I diopter concave lens to reduce the 4 diopter half lens to a 3 diopter power, the right eye image can be made sharp when the lantern slide is in position to make the left eye image sharp, and the right eye image blurred. It is a little more satisfactory to work with the 4 diopter lens, however, and to add the I diopter convex lens to the 3 diopter left lens.