There are several reasons that you should always start with a low powered objective when you are working with a specimen in the microscope. First, the lens of the high powered objective has to get extremely close to the slide in order to focus; if you are not paying close attention, you could easily crash the objective lens right through the slide, damaging the slide and/or the lens. If you begin with a lower powered objective, this is less likely to happen.
The second reason to begin with the low powered objective is that it has the widest field of view. When you use a low powered lens, you see a relatively wide area of the slide. When you increase the magnification, you see a much narrower area of the slide. A good practice would be to start with the lowest power, locate the area of the specimen that you want to see, and get that area into the middle of your field of view. Then when you increase the magnification you will be looking at the correct area of the specimen.
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