FIGURE 12.12. Image formation in (A) photopic eye; and (B) scotopic eye (B). (a-f) Paths of light rays. [After V. G. Dethier, 1963, The Physiology of Insect Senses, John Wiley and Sons, Inc. By permission of the author.]

a gradient of refractive index (decreasing from the axis outward). The lens cylinder would serve to bend light rays diverging from a point source back toward the axis, where they would form a point image. Exner proposed that photopic and scotopic ommatidia formed images in different ways. He suggested that in photopic eyes the lens cylinder had a focal length equal to its absolute length which would cause light rays to focus at the base of the cylinder, that is, on the upper end of the rhabdom (Figure 12.13A). The mosaic image formed in the eye would be of the "apposition" type. In scotopic eyes the focal length of the lens cylinder was supposedly one-half the absolute length of the lens cylinder with the result that light rays are brought to a focus, that is, the image is formed, some distance behind the cylinder, specifically halfway between the corneal surface and the center of curvature of the eye. According to Exner, this coincided with the tip of the rhabdom (Figure 12.13B). Further, in the dark-adapted position pigment granules aggregate at the distal end of the enveloping pigment cells. This permits light rays at a somewhat greater angle to the axis to be bent back toward the axis as they pass through adjacent lens cylinders. In other words, the point image formed on each rhabdom would be derived not from a single pencil of rays, as in the apposition type, but from a group of such pencils; that is, it would be formed by the superposition of light from a number of adjacent facets and is described therefore as a superposition image. In support of his ideas Exner managed to photograph objects, using the eye of the firefly Lampyris as a lens system.

During the early part of the 20th century, Exner's proposals gained acceptance and were supported by most observations that were made on insect compound eyes. However, in the 1960s, coincident with the development of more refined techniques, his ideas were subject to strong criticism, especially the concept of the lens cylinder to achieve light-ray

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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