The eyepiece ab consists of two plano-convex lenses a, b, of nearly the same focal length, and with the two convex sides facing each other.
The term Anatomy, originally employed in biological science to denote a description of the facts of structure revealed on cutting up an organism, whether with or without the aid of lenses for the purposes of magnification, is restricted in the present article, in accordance with a common modern use, to those facts of internal structure not concerned with the constitution of the individual cell, the structural unit of which the plant is composed.
Notwithstanding the fact, however, that these cells are capable of acting as very efficient lenses the explanation given by Haberlandt has not been widely accepted and evidence both morphological and physiological has been brought forward against it.
The great reflecting telescope at Dorpat was manufactured by him, and so great was the skill he attained in the making of lenses for achromatic telescopes that, in a letter to Sir David Brewster, he expressed his willingness to furnish an achromatic glass of 18 in.
To Brewster is due the merit of suggesting the use of lenses for the purpose of uniting the dissimilar pictures; and accordingly the lenticular stereoscope may fairly be said to be his invention.
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.
The lateral eyes of Scorpio consist of groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus.
In practice, however, it is not found that the presence either of a decidedly greenish-yellow colour or of numerous small bubbles interferes at all seriously with the successful use of the lenses for the majority of purposes, so that it is preferable to sacrifice the perfection of the glass in order to secure valuable optical properties.
Plate glass is, nevertheless, considerably used for the cheaper forms of lenses, where the scattering of the light and loss of definition arising from these fine striae is not readily recognized.
Hardness and Chemical Stability.-These properties contribute to the durability of lenses, and are specially desirable in the outer members of lens combinations which are likely to be subjected to frequent handling or are exposed to the weather.
If the glass is very badly annealed, the lenses made from it may fly to pieces during or of ter manufacture, but apart from such extreme cases the optical effects of internal strain are not readily observed except in large optical apparatus.
Refraction and Dispersion.-The purely optical properties of refraction and dispersion, although of the greatest importance, cannot be dealt with in any detail here; for an account of the optical properties required in glasses for various forms of lenses see the articles Lens and Aberration: Ii.
From the large masses great lenses and mirrors may be produced, while the smaller pieces are used for the production of the disks and slabs of moderate size, in which the optical glass of commerce is usually supplied.
He also had some knowledge of the properties of concave and convex lenses and mirrors in forming images.
On the strength of similar arrangements of lenses and mirrors the invention of the camera obscura has also been claimed for Leonard Digges, the author of Pantometria (1571), who is said to have constructed a telescope from information given in a book of Bacon's experiments.
About the same time Francesco Maurolico, or Maurolycus, the eminent mathematician of Messina, in his Theore y nata de Lumine et Umbra, written in 1521, fully investigated the optical problems connected with vision and the passage of rays of light through small apertures with and without lenses, and made great advances in this direction over his predecessors.
In his later book, Dioptrice (1611), he fully discusses refraction and the use of lenses, showing the action of the double convex lens in the camera obscura, with the principles which regulate its use and the reason of the reversal of the image.
Various writers on optics in the 17th century discussed the principle of the simple dark chamber alone and with single or compound lenses, among them Jean Tarde (Les Astres de Borbon, 1623); Descartes, the pupil of Kepler (Dioptrique, 1637); Bettinus (Apiaria, 1645); A.
Also several optical problems relating to lenses of various forms and their combinations for telescopic projection, rules for finding foci, &c. He does not, however, mention the camera obscura as an instrument in use, but in John Harris's Lexicon Technicum (1704) we find that the camera obscura with the arrangement called the "scioptric ball," and known as scioptricks, was on sale in London, and after this must have been in common use as a sketching instrument or as a show.
SPECTACLES, the name given to flat glasses, prisms, spherical or cylindrical lenses, mechanically adjusted to the human eyes, so as to correct defects of vision.
These prisms may be combined with concave lenses, which correct the myopia, or, since a concave lens may be considered as composed of two prisms united at their apices, the same effect may be obtained by making the distance between the centres of the concave lenses greater than that between the centres of the pupils.
Again, to obviate the necessity for excessive convergence of the eyes so common in hypermetropia, the centre of the pupil should be placed outside the centre of the corrective convex lenses; these will then act as prisms with their bases inwards.
- Biconcave, biconvex and concavo-convex (meniscus) lenses are employed in ophthalmic practice in the treatment of errors of refraction.
Until recently these spherical lenses were numbered in terms of their focal length, the inch being used as the unit.
Concave Lenses are used in the treatment of myopia or shortsight.
Concave lenses should never be used for work within the far point; but they may be used in all cases to improve distant vision, and in very short-sighted persons to remove the far point so as to enable fine work such as sewing or reading to be done at a convenient distance.
The weakest pair of concave lenses with which one can read clearly test types at a distance of 18 ft.
Such cases should be treated with convex lenses, which should be theoretically of such a strength as to fully correct the hypermetropia.
Becomes impossible, convex lenses should be used for reading of such a strength as to enable the eye to see clearly about 8 in.
This condition may be cured completely, or greatly improved, by the use of lenses whose surfaces are segments of cylinders.
Here, in order to fulfil the purposes of the previous models, the distance of the centres of the lenses from each other should only slightly exceed the tangent of sun's diameter X focal length of lenses.
Savary dwells on the difficulty both of procuring lenses sufficiently equal in focus and of accurately adjusting and centring them.
This construction would give all the advantage of the younger Dollond's object-glass micrometer, and more than its sharpness of definition, without liability to the systematic errors which may be due to want of homogeneity of the object-glass; for the lenses will not be turned with respect to each other, but, in measurement, will always have the same relation in position angle to the line joining the objects under observation.
Focus, would correspond with 2" of arc. But, after all, this is no practical difficulty, for screws can be used to separate the lenses, and, by these screws, as in a Gascoigne micrometer, the separation of the lenses can be measured; or we can have scales for this purpose, read by microscopes, like the Troughton 1 circles of Piazzi or Pond, or those of the Carey circle, with almost any required accuracy.
The setting of both lenses symmetrically would, under such circumstances, be very tedious.
In this construction the lenses are much closer together and the diaphragm for the eye is much farther from the lenses than in Ramsden's eye-piece.
This lens is divided and mounted like a heliometer objectglass; the separation of the lenses produces the required double image, and is measured by a screw.
Pp. 199-209) the rays from the object-glass pass successively through lenses as follows: The lens b is divided, and one of the segments is moved by a micrometer screw.
Struve (Description de l'Observatoire ' Central de Pulkowa, pp. 196, 197) adds a few remarks to Steinheil's description, in which he states that the images have not all desirable precision - a fault perhaps inevitable in all micrometers with divided lenses, and which is probably in this case aggravated by the fact that the rays falling upon the divided lens have considerable convergence.
He appears soon to have found that single lenses of very short focus were preferable to the compound microscopes then in use; and it is clear from the discoveries he made with these that they must have been of very excellent quality.
His skill as a working lapidary was very great; and he prepared a number of lenses of garnet and other precious stones, which he preferred to the achromatic microscopes of the time.
The coloured borders seen in the images produced by simple lenses are due to dispersion.
He had welcomed with a little essay called Dissertatio cum Nuncio Sidereo Galileo's first announcement of celestial novelties; he now, in his Dioptrice (Augsburg, 1611), expounded the theory of refraction by lenses, and suggested the principle of the "astronomical" or inverting telescope.
Such stones have been occasionally cut as lenses for microscopes, being recommended for such use by their high refractivity, weak dispersion and great hardness.
By introducing the concave grating which (see Diffraction Of Light, § 8) allows US to dispense with all lenses, Rowland produced a revolution in spectroscopic measurement.
Essentially it consists in an optical system of lenses and mirrors, or mirrors alone, the upper part of which projects from cover, or from the deck of a submarine, while the observer looks into the lower end, receiving an image of the surrounding country or sea by reflection down a tube.
From the beginning of the 20th century, however, the practical introduction of submarine navigation brought about the development of new elaborate periscopes of great length and provided with an optical system of lenses, which were built into the structure of the submarine.
The two achromatic lenses, C and D, bring the rays to a focus on the plane surface of the large lens, E, forming an image there.
Trans., 1758), describing the experiments that led him to the achievement with which his name is specially associated, the discovery of a means of constructing achromatic lenses by the combination of crown and flint glasses.
Early in 1757 he succeeded in producing refraction without colour by the aid of glass and water lenses, and a few months later he made a successful attempt to get the same result by a combination of glasses of different qualities (see Telescope).