Why do you have to watch from the side when changing objecives


It is important to watch from the side to make sure the objective you are moving into position is going to clear the slide,not hit the slide.
If you mean at the side of the microscope, well, it is to know if what type or kind of an objective are you using, if it is a scanner, HPO or LPO, and to aware if you are changing the objectives.
If you are talking about microscope
it is so that you might not break the glass slide
when you are changing or focusing the lens/objectives
For you to know if it is on the right side or not.
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Because the slide might break when changing the objectives to HPO when the coarse adjustment knob is not adjusted upward.
Because of the mirror 
Because as you turn the nosepiece the objective might hit the glass slide bearing the specimen which may cause a scratch in the lense of the objective

Why do you have to watch from the side when changing objectives ? When adjusting the lenses, glass or lens will break when looking in the microscope. The glass is so fragile it can easily crack. We then look from the side to keep it from splitting as we change the objectives.

Because of the mirror

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