You probably noticed by now that it can be tedious to zoom in and out; clicking on the zoom tools and then moving the scrollbars around to find a particular part of your disk is just too slow.
There is a better solution, and we call it the zoom tool. It's
the tool that looks like a plain old magnifying glass:
Click on this tool now. Notice that nothing happened, other than that the highlight disappeared around the arrow above it and appeared around it. That highlight indicates which primary tool is active; in this case, the zoom tool is active, and the arrow --- the select tool --- is not.