LISTENING 2 “ART CLASS ”
Narrator Listen to part of a lecture in an art class.
Symmetry is a concept that, yes, is expressed in the graphic arts, but to understand its fundamental nature, we must go beyond art. We find symmetry in nature, it reverberates in music, translates into choreography for dance, and . . . underlies basic mathematical formulas. But I’m getting ahead of myself. Let’s begin with a dictionary definition of symmetry. And I’m reading here from the Amencan Heritage Dictionary of the English Language. Symmetry is “exact correspondence of form and constituent configuration on opposite sides of a dividing line or plane or about a center or an axis.” And it’s also identified in the same source as “beauty as a result of balance or harmonious arrangement.” So we experience beauty and harmony when symmetry of form is expressed, and the form may be interpreted by any of the senses as, uh, harmonious. But in this class we’re going to focus on symmetry in the visual arts, and that’s symmetry in a visual plane.
Let’s just look at some examples, in general, there are four types of symmetry in a plane, and a pattern is symmetrical if there’s at least one type of symmetry. So, let me show you the four types since it’s much easier to understand when you see them.
This is rotation symmetry. For this example, I used the letter R, but any object could have been used. And in the rotation, the object, in this example, the tetter R, is turned around a center. In this case, there’s a right angle, but any angle could have been selected.
Reflection is… wait a minute. Okay, here’s the slide
Reflection is what we see in a mirror, so every reflection has a mirror line. A reflection of the letter R is a mirror image or a backwards teller R. So. unlike the rotation around a circle, this type of sysmetry flips the object over.
Here’s a translation. To translate an object means that we move it, but we do it without rotation or reflection. It’s simply placed somewhere else on the plane. And for our purposes, we’re talking about a flat plane. So. in this example, uh, we just moved it over a little bit.
Okay, this is my last example of symmetry and it’s referred to as glide reflection. This is the most complex type of symmetry because it involves two steps instead of one. A glide reflection is a combination of a reflection and a translation along the direction of the minor line. So, uh, you can see the two steps here. First, we flip it over and then we move it somewhere else on the plane.
Of course these concepts can be generalized to include spatial symmetry as well. But, symmetry on a flat plane involves positioning ail points around the plane so their positions in relationship to each remain constant . . . although their absolute positions may be subject to change. To put it in simple terms, if an object looks the same to you after you spin it around, flip it over, or look at it in a minor, then that object probably has symmetry.
Symmetry is such a fundamental organizing principle that an object with symmetry can be identified … without our being able to see the, uh,… the entire object. Our brains somehow piece together the missing pieces to form a symmetrical whole. Which is really rather extraordinary, when you think about it. At some very basic level, symmetry may be part of the way that we .. that we organize our thinking. And of course, that would explain why it’s so pleasing.
So now let’s return to symmetry in art. Symmetry stands out and attracts attention. It’s the system of organization for patterns. But what is a pattern? A pattern has three characteristics a system for organization, and like we said before, this is often symmetry, but a pattern also has a basic unit, that is, uh. it’s an object that’s the smallest discrete part of the image. As you’ll recall from the types of symmetry that we discussed, the letter R was the basic unit. Okay, finally, a pattern has repetition, which can be the repetition of a unit or a group of unite. And, uh, this repetition, in much of art, this repetition is arranged symmetrically.
Just look around the classroom. Look at the tiles on the floor. Here you see a symmetrical design with four repeating tiles. The tiles were not placed at random. There’s a pattern here with all three characteristics of a pattern first, there’s a unit, a basic unit, of four tiles; second, there’s repetition of the tiles with solid tiles surrounding them; and, uh, third, there’s symmetry… within the four tiles, which to be specific, looks like rotation symmetry to me.
Now, for your studio assignment, I want you to draw a pattern that has as its organizing principle, a symmetrical design. It can be either in color or in black and white, but it must fit on a piece of standard 81/2 by 11 -inch paper. On a second sheet of paper, I want you to identify the type of symmetry that you used. Perhaps some of you will want to experiment with several types of symmetry, but if you do, please be sure to identify each of them clearly in your narrative. For this first effort, I recommend that you stick to something relatively simple, like the tile floor. So, when you come to class next week, be ready to share your design with three other people in a group. Then I’ll collect them at the end of the hour.
LISTENING 3 “BIOLOGY CLASS”
Nanator. Listen to part of a lecture in a biology class.
By studying the fossil record we can read the history of life on Earth. Interestingly enough, it appears that there are long periods in which not very much change occurs; then sporadic brief periods in which there are mass extinctions of species followed by diversification of the groups that survived. How docs this happen? Well, sometimes a habitat is destroyed or the environment changes. Did you know that if the temperature of the ocean falls by even a few degrees, many species will die? Incredible, isn’t it? Or. even when the environment is relatively stable, biological conditions can change when other species evolve in different directions. For example, let’s see, when a similar species evolves by developing a shell, then the related species without shells may be more vulnerable to predators and could become extinct as a resurt of changes in the other species. So you can see that extinction IS a natural consequence of history. It’s, well, inevitable. But sometimes mass extinctions occur and most of the known species are lost. And this is very different.
Let me mention two such mass extinctions. First, the Permian mass extinction, which occurred about 250 minion years ago. According to fossil records, more than 90 percent of the marine spedes and about 30 percent of the orders of insects pefished. Then about 65 million years ago, the Cretaceous mass extinction claimed more than half of the marine species and many terrestrial species of plants and animals, including the dinosaurs.
So what causes mass extinction? This isn’t an easy question to answer. You see, it’s obvious from the fossil records that species exist during a certain geological time period, and then, they disappear, and we have said evidence for that. But why they disappear is, well, more speculative. In the Permian, several extreme conditions miay have converged, including the merging of the continents into one large land mass. As you can imagine, such a radical change in the distribution of land and water would have disturbed habitats and caused the climate to change. There’s also evidence that volcanic activity during this period may have produced enough carbon dioxide to cause global warming, which in tum would have affected the temperature and depth of the oceans, and it, and I’m referring here to global warming, so it probably also caused the oxygen levels in the oceans to decrease. All of these conditions could have converged to extinguish an enormous number of species at the same time. That’s mass extinction.
And, a similar set of conditions may also have contributed to the mass extinction in the Cretaceous period as well. We can gather data that convinces us about continental drift…that it occurred along with receding seas along the continental coastlines. In addition, we know that cooler climate was probably the result, at least in part, of . . . increased volcanic eruptions, and these eruptions probably released enough material into the atmosphere to block the sunlight. Having said all of that, many scientists now favor a very different hypothesis. They theorize that maybe a large asteroid collided with the Earth. Advocates of the so-called impact hypothesis speculate that there were two events that caused the mass extinction. First, the impact probably caused a fire storm of such proportion that most of the life in North America would have been decimated within minutes. Second, they postulate that an enormous doud of fallout could have blocked out the sunlight and … that the impact was. in fact, large enough to … darken the Earth … and we’re talking about months or even years. So the result… of the darkness, I mean … that would have caused a reduction in photosynthesis, which, in turn, would have created a disruption in the food chain. Now, such a disruption would have affected many species.
So the advocates of the impact hypothesis … they put forward evidence that a thin layer of clay, rich in indium deposits, uh, can be found in the geologic material that separates the Mesozoic and the Cenozok: eras—precisely the time period for the Cretaceous mass extinction. So what’s special about this day? Wei, indium is a very rare element on Earth, but ifs quite common in meteorites and other extraterrestrial debris thafs been analyzed. So, it’s possible that this sediment is the remains of the impact. The fact that there was more serious damage to the species in the Western Hemisphere could also be explained by the point of impact, and the fact that the dust cloud could have caused more acidic precipitation nearer the area of impact. Or. there may have been a number of calamities that converged simultaneously, disrupting planetary balances.
But whatever the cause or causes, the fact remains that the mass extinctions occurred, and they influenced the biological diversity of our planet in profound ways. The species that survived, whether because they had genetic advantages or because they were fortunate enough to be farther from the catastrophes… these species became the ancestors of the species that have played important roles in biological evolutionary history.