The lateral eyes are in Limulus " compound eyes," that is to say, consist of many lenses placed close together; beneath each lens is a complex of protoplasmic cells, in which the optic nerve terminates.
If not, the substitution of an achromatic lens will be of no advantage.
R, The retinal portion of the same which, owing to the infolding, lies between gl, the corneagen or lens-forming portion, and pr, the post-retinal or capsular portion or fold.
The curved surfaces take the place of fl e the lens in fig.
Watase has shown, in a very convincing way, how by deepening the pit-like set of cells beneath a simple lens the more complex ommatidia of the compound eyes of Crustacea and Hexapoda may be derived from such a condition as that presented in the lateral eyes of Limulus and Scorpio.
(For details the reader is referred to Watase (11) and to Lankester and Bourne (5).) The structure of the central eyes of Scorpio and spiders and also of Limulus differs essentially from that of the lateral eyes in having two layers of cells (hence called diplostichous) beneath the lens, separated from one another by a membrane (figs.
Such a plate has the power of a condensing lens, and gives an illumination out of all proportion to what could be obtained without it.
If the source be a point or a line, and a collimating lens be used, the incident waves may be regarded as plane.
The irregularity of spacing has thus the effect of a convex lens, which accelerates the marginal relatively to the central rays.
As a source of light he used sunshine passing through a very small hole perforated in a metal plate, or condensed by a lens of short focus.
In observing the bands he received them at first upon a screen of finely ground glass, upon which a magnifying lens was focused; but it soon appeared that the ground glass could be dispensed with, the diffraction pattern being viewed in the same way as the image formed by the object-glass of a telescope is viewed through the eye-piece.
A lady's hand glass) behind the lens and inclined at an angle of 45° to the horizon so as to reflect Mirror the rays of light vertically downwards, we can produce >» on a horizontal sheet of Image with Mirror paper an unperverted image FIG.
The response to the action of light in diatropic leaves is, according to Haberlandt, due to the presence of epidermal cells which are shaped like a lens, or with lens-shaped thickenings of the cuticle, through which convergence of the light rays takes place and causes a differential illumination of the lining layer of protoplasm on the basal walls of the epidermal cells, by which the stimulus resulting in the orientation of the leaf is brought about.
He was the inventor of the stage-micrometer, and of a form of heliometer; and in 1816 he succeeded in constructing for the microscope achromatic glasses of long focus, consisting of a single lens, the constituent glasses of which were in juxtaposition, but not cemented together.
On the surface of the carapace there are in both animals a pair of central eyes with simple lens and a pair of lateral eyetracts, which in Limulus consist of closely-aggregated simple eyes, forming a " compound" eye, whilst in Scorpio they present several AC separate small eyes.
The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus.
They are at the same time both optic nerve-end cells, that is to say, retina cells, and corneagen cells or secretors of the chitinous lens-like cornea.
The upper layer is the corneagen and secretes the lens, the lower is the retinal layer.
The mass of soft cell-structures beneath a large lens of a central eye is called an " ommatoeum."
(From Lankester, loc. cit.) forming a star-like chitinous centre in section, each lateral eye of Scorpio has several rhabdoms of five or less rhabdomeres, indicating that the Limulus lateral eye-unit is more specialized than the detached lateral eyelet of Scorpio, so as to present a coincidence of one lens with one rhabdom.
H, Epidermic cell-layer; mes, mesoblastic connective tissue; n, nerves; II, III, IV, V, depressions of the epidermis in each of which a cuticular lens will be formed.
- Section through a portion of the lateral eye of Limulus, showing three ommatidia - A, B and C. hyp, The epidermic cell-layer (so-called hypodermis), the cells of which increase in volume below each lens, 1, and become nerve-end cells or retinula-cells, rt; in A, the letters rh point to a rhabdomere secreted by the cell rt; c, the peculiar central spherical cell; n, nerve fibres; mes, mesoblastic skeletal tissue; ch, chitinous cuticle.
A, Early condition before the lens is deposited, showing the folding of the epidermic cell-layer into three.
G, Line separating lens from the lens-forming or corneagen cells of the epidermis.
In the case of a single lens of glass with the most favourable curvatures, Sf is about equal to a 2 f, so that a 4 must not exceed off.
For a lens of 3 ft.
When parallel rays fall directly upon a spherical mirror the longitudinal aberration is only about one-eighth as great as for the most favourably shaped single lens of equal focal length and aperture.
If, further, on leaving the grating the light be received by a focusing lens, e.g.
The capital of the province was Arras, and the other important places were Saint-Omer, Bethune, Aire, Hesdin, Bapaume, Lens, Lillers, Saint-Pol and SaintVenant.
Flint glass particularly, which appeared quite satisfactory when viewed in small pieces, was found to be so far from homogeneous as to be useless for lens construction.
A crystal lens, turned on the lathe, was discovered by Layard at Nimrud along with glass vases bearing the name of Sargon; this will explain the excessive minuteness of some of the writing on the Assyrian tablets, and a lens may also have been used in the observation of the heavens.
CAMERA OBSCURA, an optical apparatus consisting of a darkened chamber (for which its name is the Latin rendering) at the top of which is placed a box or lantern containing a convex lens and sloping mirror, or a prism combining the lens and mirror.
If we hold a common reading lens (a magnifying lens) in front of a lamp or some other bright object and at some distance from it, and if we hold a sheet of paper vertically at a suitable distance behind the lens, we see depicted on the paper an image of the lamp. This image is inverted and perverted.
In the above argument the whole space between the object and the lens is supposed to be occupied by matter of one refractive index, and X represents the wave-length in this medium of the kind of light employed.
The function of a lens in forming an image is to compensate by its variable thickness the differences of phase which would otherwise exist between secondary waves arriving at the focal point from various parts of the aperture.
If we suppose the diameter of the lens to be given (2R), and its focal length f gradually to increase, the original differences of phase at the image of an infinitely distant luminous point diminish without limit.
But, as we have seen, such an error of phase causes no sensible deterioration in the definition; so that from this point onwards the lens is useless, as only improving an image already sensibly as perfect as the aperture admits of.
A similar argument may be applied to find at what point an achromatic lens becomes sensibly superior to a single one.
Calculation shows that, if the aperture be s in., an achromatic lens has no sensible advantage if the focal length be greater than about II in.
In the absence of a heliostat it is more convenient to obtain a point of light with the aid of a lens of short focus.
Lens, Cuticular lens.
The central eyes are " simple eyes," that is to say, have a single lens, and are hence called " monomeniscous."