Perceptions of the visible world were greatly altered by the invention of photography in the middle of the nineteenth century.
In particular, and quite logically, the art of painting was forever changed, though not always in the ways one might have expected.
The realistic and naturalistic painters of the mid- and late-nineteenth century were all intently aware of photography—as a thing to use, to learn from, and react to.
Unlike most major inventions, photography had been long and impatiently awaited.
The images produced by the camera obscura(暗盒), a boxlike device that used a pinhole or lens to throw an image onto a ground-glass（毛玻璃） screen or a piece of white paper, were already familiar—the device had been much employed by topographical artists like the Italian painter Canaletto in his detailed views of the city of Venice.
What was lacking was a way of giving such images permanent form.
This was finally achieved by Louis Daguerre (1787-1851), who perfected a way of fixing them on a silvered copper plate.
His discovery, the “daguerreotype,” was announced in 1839.
A second and very different process was patented by the British inventor William Henry Talbot (1800-1877) in 1841.
Talbot’s “calotype” was the first negative-to-positive process and the direct ancestor of the modern photograph.
The calotype was revolutionary in its use of chemically treated paper in which areas hit by light became dark in tone, producing a negative image.
This “negative,” as Talbot called it, could then be used to print multiple positive images on another piece of treated paper.
The two processes produced very different results.
The daguerreotype was a unique image that reproduced what was in front of the camera lens inminute, unselective detail and could not be duplicated.
The calotype could be made in series, and was thus the equivalent of an etching or an engraving.
Its general effect was soft edged and tonal.
One of the things that most impressed the original audience for photography was the idea of authenticity.
Nature now seemed able to speak for itself, with a minimum of interference.
The title Talbot chose for his book, The Pencil of Nature (the first part of which was published in 1844), reflected this feeling.
Artists were fascinated by photography because it offered a way of examining the world in much greater detail.
They were also afraid of it, because it seemed likely to make their own efforts unnecessary.
Photography did indeed make certain kinds of painting obsolete—the daguerreotype virtually did away with the portrait miniature.
It also made the whole business of making and owning images democratic.
Portraiture, once a luxury for the privileged few, was suddenly well within the reach of many more people.
In the long term, photography’s impact on the visual arts was far from simple.
Because the medium was so prolific, in the sense that it was possible to produce a multitude of images very cheaply, it was soon treated as the poor relation of fine art, rather than its destinedsuccessor.
Even those artists who were most dependent on photography became reluctant to admit that they made use of it, in case this compromised their professional standing.
The rapid technical development of photography—the introduction of lighter and simpler equipment, and of new emulsions（感光乳剂） that coated(覆盖，涂，镀膜) photographic plates, film, and paper and enabled images to be made at much faster speeds—had some unanticipated consequences.
Scientific experiments made by photographers such as Eadweard Muybridge (1830-1904) and Etienne-Jules Marey (1830-1904) demonstrated that the movements of both humans and animals differed widely from the way they had been traditionally represented in art.
Artists, often reluctantly, were forced to accept the evidence provided by the camera.
The new candid photography—unposed pictures that were made when the subjects were unaware that their pictures were being taken—confirmed these scientific results, and at the same time, thanks to the radical cropping (trimming) of images that the camera often imposed, suggested new compositional formats.
The accidental effects obtained by candid photographers were soon being copied by artists such as the French painter Degas.
Sometime after midnight on February 8,1969, a large, bright meteor entered Earth’s atmosphere and broke into thousands of pieces, plummeted to the ground, and scattered over an area 50 miles long and 10 miles wide in the state of Chihuahua in Mexico.
The first meteorite from this fall was found in the village of Pueblito de Allende.
Altogether, roughly two tons of meteorite fragments were recovered, all of which bear the name Allende for the location of the first discovery.
Individual specimens of Allende are covered with a black, glassy crust that formed when their exteriors melted as they were slowed by Earth’s atmosphere.
When broken open, Allende stones are revealed to contain an assortment of small, distinctive objects, spherical or irregular in shape and embedded in a dark gray matrix (binding material), which were once constituents of the solar nebula—the interstellar cloud of gas and dust out of which our solar system was formed.
The Allende meteorite is classified as a chondrite.
Chondrites take their name from the Greek word chondros—meaning “seed”—an allusion to their appearance as rocks containing tiny seeds.
These seeds are actually chondrules: millimeter-sized melted droplets of silicate material that were cooled into spheres of glass and crystal.
A few chondrules contain grains that survived the melting event, so these enigmatic chondrules must have formed when compact masses（质密） of nebular dust were fused at high temperatures—approaching 1,700 degrees Celsius—and then cooled before these surviving grains could melt.
Study of the textures of chondrules confirms that they cooled rather quickly, in times measured in minutes or hours, so the heating events that formed them must have been localized.
It seems very unlikely that large portions of the nebula were heated to such extreme temperatures, and huge nebula areas could not possibly have lost heat so fast.
Chondrules must have been melted in small pockets of the nebula that were able to lose heat rapidly.
The origin of these peculiar glassy spheres remains an enigma.
Equally perplexing constituents of Allende are the refractory inclusions: irregular white masses that tend to be larger than chondrules.
They are composed of minerals uncommon on Earth, all rich in calcium, aluminum, andtitanium, the most refractory (resistant to melting) of the major elements in the nebula.
The same minerals that occur in refractory inclusions are believed to be the earliest-formed substances to have condensed out of the solar nebula.
However, studies of the textures of inclusions reveal that the order in which the minerals appeared in the inclusions varies from inclusion to inclusion, and often does not match the theoretical condensation sequence for those metals.
Chondrules and inclusions in Allende are held together by the chondrite matrix, a mixture of fine-grained, mostly silicate minerals that also includes grains of iron metal and iron sulfide.
At one time it was thought that these matrix grains might be pristine nebular dust, the sort of stuff from which chondrules and inclusions were made.
However, detailed studies of the chondrite matrix suggest that much of it, too, has been formed by condensation or melting in the nebula, although minute amounts of surviving interstellar dust are mixed with the processed materials.
All these diverse constituents are aggregated together to form chondritic meteorites, like Allende, that have chemical compositions much like that of the Sun.
To compare the compositions of a meteorite and the Sun, it is necessary that we use ratios of elements rather than simply the abundances of atoms.
After all, the Sun has many more atoms of any element, say iron, than does a meteorite specimen, but the ratios of iron to silicon in the two kinds of matter might be comparable.
The compositional similarity is striking.
The major difference is that Allende is depleted in the most volatile elements, like hydrogen, carbon, oxygen, nitrogen, and the noble gases, relative to the Sun.
These are the elements that tend to form gases even at very low temperatures.
We might think of chondrites as samples of distilled Sun, a sort of solar sludge（泥污，沉淀物）from which only gases have been removed.
Since practically all the solar system’s mass resides in the Sun, this similarity in chemistry means that chondrites have average solar system composition, except for the most volatile elements;
they are truly lumps（肿块，傻大个） of nebular matter, probably similar in composition to the matter from which planets were assembled.