shape and appearance, is one of the leaves of chaff that surround a grain of wheat. It is of an elongated form-not oval, but pointed at one end; and the similarity between the two objects goes still farther, for both have a bright external appearance. But here the resemblance ends; and what a contrast is revealed by an examination of the two objects under the microscope! The piece of chaff presents a uniform glazed surface, whilst in the eye of the Bee, which is much darker in colour, the brightness referred to arises from its peculiar structure; in fact, it is owing to the presence of about 3500 small but perfect hexagonal lenses fitting closely together, and disposed in regular rows over the whole circumference. You will not be surprised, after this statement, to hear that the compound eye of a Bee is one of the most exquisitely constructed instruments in nature (we are almost tempted to say the most exquisite), and one which, small though it be, displays the power and wisdom of the Omnipotent in as striking a manner as do His most imposing and majestic works. Let us therefore solicit your careful attention, whilst we endeavour to describe the details of its wonderful structure; and we can promise that you will be well requited for the trouble of accompanying us in the investigation. In order to afford some idea of the general character and operation of one of these compound eyes, we shall compare it to a bundle of telescopes (3500, remember!), so grouped together that the large terminal lenses present an extensive convex surface, whilst, in consequence of the decreasing diameter of the instruments, their narrow ends meet and form a smaller concentric curve. Now, if you can imagine it possible to look through all these telescopes at one glance, obtaining a similar effect to that of the stereoscope, you will be able to form some conception of what is probably the operation of vision in the Bee. This comparison, however, presents but a crude and imperfect idea of the organ in question, and we shall now accurately describe one of these "telescopes," as we have popularly termed them. Each of the eyelets or "ocelli," which, aggregated, constitute the compound eye of a Bee, is itself a perfect instrument of vision, consisting of two remarkably formed lenses, namely an outer "corneal" lens (Pl. IV. fig. 1, a a a), and an inner or "conical" lens (6). The "corneal" lens (a) is a hexahedral or six-sided prism, and it is the assemblage of these prisms that forms what is called the "cornea" of the compound eye. This “cornea” may easily be peeled off, and if the whole, or a portion, be placed under the microscope, the grouping of the beautiful lenses becomes distinctly visible. In Pl. IV. fig. 2, which represents a portion of the cornea magnified, a little of the subjacent layer of colouring-matter (pigmentum nigrum), of which we shall speak hereafter, has also been re tained, and is distinguishable behind some of the lenses. But, stay! we must not yet part company with the corneal lens of the Bee's eyelet; for, on closer investigation, we shall perceive that it is not a simple, but a compound lens,—a fact of considerable importance, that has, we believe, been overlooked by physiologists. It is composed of two plano-convex lenses (Pl. IV. fig. 1, 71, 72) (that is, as you doubtless know, lenses having a plane and a convex surface) of different densities or refracting powers, and the plane surfaces of these lenses being adherent, it follows that the prismatic corneal lens is a compound double convex lens*. The effect of this arrangement is, that if there should be any aberration or divergence of the rays of light during their passage through one portion of the lens, it is rectified in its transit through the other. Now it is nothing new to find in the eye of an animal lenses of different densities, but we do not recollect ever having heard of any other instance where one compound lens has been found consisting of two adherent ones of this descriptiont. How remarkable, then, that we should discover such a phenomenon in so humble an animal as the Bee! Aye, reader; and how remarkable, too, that we should find such a contrivance adopted by man in the construction of what he at present considers the most perfect microscopic lens! * We believe the credit of this discovery is due to Dr. J. B. Hicks. It is not unlikely that the eyes of other insects are similarly constructed. |