(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.
If not, the substitution of an achromatic lens will be of no advantage.
Its object is a practical one, to determine by scientific considerations the shape of lens best adapted to improve the capabilities of the telescope, which had been invented not long before.
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.
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.
Ginger, coiffed and styled by the best, looked ready for a fashion photographer's lens while continually rolling her eyes with disdain toward her sister-in-law.
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.
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 irregularity of spacing has thus the effect of a convex lens, which accelerates the marginal relatively to the central rays.
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 curved surfaces take the place of fl e the lens in fig.
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.
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."
- 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.
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.
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.
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 the source be a point or a line, and a collimating lens be used, the incident waves may be regarded as plane.
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.
He was well acquainted with the use of magnifying glasses and suggested a kind of telescope for viewing the moon, but does not seem to have thought of applying a lens to the camera.
The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus.
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.
G, Line separating lens from the lens-forming or corneagen cells of the epidermis.
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.
If, further, on leaving the grating the light be received by a focusing lens, e.g.
2nd Image Object Mirror Image without Lens camera obscura, which was extensively used in sketching from nature before the introduction of photography, although it is now scarcely to be seen except as an interesting side-show at places of popular resort.
Generally the mirror and lens are combined into a single piece of worked glass represented in section in fig.
The account is not very clear, but seems to imply the use of a concave mirror rather than a lens, which might be suggested by the word orbem.
Nothing is said about the use of a lens or of solar observations.
In the Korn apparatus the light from a Nernst electric lamp is concentrated to a point by means of a lens on the original picture, which is wound on a glass cylinder in the shape of a transparent photographic film.
Sir David Brewster modified his apparatus by moving the object-box and closing the end of the tube by a lens of short focus which forms images of distant objects at the distance of distinct vision.
RHIPIDOGL0ssA.-Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single.
These dorsal eyes are very perfect in elaboration, possessing lens, retinal nerve-end cells, retinal pigment and optic nerve.
Curiously enough, however, they differ from the cephalic Molluscan eye in the fact that, as in the vertebrate eye, the filaments of the optic nerve penetrate the retina, and are connected with the re surfaces of the nerve-end cells nearer the lens instead of with the opposite end.
Kidneys, pancreas and the thyroid gland, also in muscle-plasma; " crystalline," a globulin occurring in two forms a and /3, is found in the lens of the eye; " egg-globulin " and " lactoglobulin " occur respectively in the white of egg and in milk.
The central eyes are " simple eyes," that is to say, have a single lens, and are hence called " monomeniscous."
Lens, Cuticular lens.
In experiment under ordinary circumstances it makes no difference whether the collecting lens is in front of or behind the diffracting aperture.
If we suppose the focal length to be 66 ft., a single lens is practically perfect up to an aperture of 1 .
For a lens of 3 ft.
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 absence of a heliostat it is more convenient to obtain a point of light with the aid of a lens of short focus.