"Art History Class" What is the main topic of this lecture?

Narrator: Listen to part of a lecture in an art history class.
Professor:
We know that the ChineSe had been aware of basic photographic principles as early as the fifth  Q12
century B.c., and Leonardo da Vinci had experimented with a dark room in the 1500s, but it was
a number of discoveries in chemistry during the eighteenth century that, uh, accelerated the
development of modern photography. The discovery that silver salts were light sensitive led to
experimentation with images of light on a... asurface that had been coated with silver. Often glass was
used in the early images. But the problem was that these images were ephemeral—fading after only a
short time. Some of the chemists who worked with them called them fairy pictures, and considered
them, uh, that they were only momentary creations, uh, that they would disappear.
    Okay. How to fix the image permanently was one of the most important, uh, challenges... of the
early photographer chemists. In France, in about 1820, Nicephore Niepce discovered a method for
fixing the image after a long exposure time, oh, probably eight hours. So, although his work was
considered interesting, it was, uh, uh, largely dismissed for... as impractical. Nevertheless, one of his
associates, Louis Daguerre, managed to find a way to, uh, reduce... the exposure time to less than
twenty minutes. So the story goes, in 1835, Daguerre was experimenting with some exposed plates,
and he put a couple of them into his chemical cupboard, so a few days later, he opened the cupboard,
and, uh, to his surprise, the latent images on the plates had developed. At first, he couldn’t figure out  Q14
why, but eventually, he concluded that this must have occurred as a result of mercury vapor... from a
broken thermometer that was also in the, uh, enclosed in the cupboard. Supposedly from this fortunate
accident, he was able to invent a process for developing latent images on... on exposed plates.
    The process itself was somewhat complicated. First, he exposed copper plates to iodine which
released fumes of, uh, of light-sensitive silver iodide. These copper plates were used to capture the
image, and by the way, they had to be used almost immediately after their exposure to the iodine. So,
the image on the plate was then exposed to light for ten to twenty minutes. The plate was developed
over mercury heated to about 75 degrees centigrade, which.., that caused the mercury to amalgamate with the silver. Now here’s the ingenious part—he then fixed the image in a warm solution of common  Q15
salt, but later he began Using sodium sulphite. Anyway, after he rinsed the plate in hot distilled water, a
white image was left permanently on the plate. And the quality was really quite amazing.
    But, um... the process had its limitations. First, the images couldn’t be reproduced, so each one  Q13
was a unique piece, and that, uh, greatly increased the cost of photography. Second, the image was
reversed, so the subjects would actually see themselves as though they were looking in a mirror,
although, uh, in the case of portraits, the fact that people were accustomed to seeing themselves in a
mirror made this less... this problem less urgent than some of the others. Nevertheless, some
photographers did point their cameras at a mirrored reflection of the image that they wanted to capture so that
the reflection would be reversed, and a true image could be produced. Okay. Third, the chemicals and  Q16
the fumes that they released were highly toxic, so photography was a very dangerous occupation.
Fourth, the surface of the image was extremely fragile and... had to be protected, often under glass,  Q13
so they didn’t disintegrate from being... from handling. The beautiful cases that were made to hold the
early images became popular not only for aesthetic purposes but, uh, but also for very practical
reasons. And finally, although the exposure time had been radically reduced, it was still.., inconveniently
long... at twenty minutes, especially for portraits, since people would have to sit still in the sun for that
length of time. Elaborate headrests were constructed to keep the subjects from moving so that the
image wouldn’t be ruined, and, uh, many people simply didn’t want to endure the discomfort.
    But, by the mid 1800s, improvements in chemistry and optics had resolved most of these issues.  Q17
Bromide as well as iodine sensitized the plates, and some photographers were even using chlorine in
an effort to decrease exposure time. The... the portrait lens was also improved by reducing the size of
the opening, and limiting the amount of light that could enter, so the exposure time was about twenty
seconds instead of twenty minutes. And negative film had been introduced in France, sorry, in England,
and negatives permitted the production of multiple copies from a single image. So, photography was on
its way to becoming a popular profession and pastime.