【正确答案】The two major systems of the laboratory microscope are the illuminating system and the imaging system. The illuminating system has a light source that can be controlled to let more or less light pass through the specimen you're viewing when you move a lever on the side of the microscope. The imaging system is actually a magnification feature, which can be calibrated by using the other lever to change powers. When you look through a higher power, the image appears to be larger, and conversely, when you look through a lower power, it appears to be smaller. What really means is when you use a higher magnification, the image actually shows you a smaller part of the specimen because it's enlarged. Now when you switch powers, the lenses will focus automatically, but the big knob will allow you to make a rough adjustment, and the small knob will let you make a more detailed adjustment, if you want.

【答案解析】[听力原文] All right, now that you all have microscopes at your tables, I want to explain how they work and how to use them. First of all, you should know that they are compound microscopes so they can magnify objects up to 1000 times their size. These microscopes have two systems an illuminating system and an imaging system. You'll see that the illuminating system has a light source built in, and you can control it by adjusting the lever on the side. Why is this important'? Well, the specimen must be pretty thin, or let's say, transparent enough to let light pass through it. So the light source controls the amount of light that passes through the specimen. Okay. The other system provides magnification. Use this lever to switch powers. So, when you switch to a higher power, you can see a larger image, and when you switch to a lower power, you see a smaller image. But I should remind you that the field of view is smaller at a higher power. In other words, at a higher magnification, you can see a larger image of a smaller area. Okay. So what about the focus? Well, these microscopes are parfocal, and that means you should usually don't have to refocus when you switch to a higher or lower power of magnification. But there are two adjustment knobs, the larger one for coarse adjustment and the smaller one for fine adjustment just in case.