If you're talking about Light's explanation of his plan in Episode 36 (1.28), see spoiler below.
If you're talking about Episode 35 (Malice), where we see Mikami check the Death Note with a microscope and call Takada to say that he confirmed something... Even without knowing Light's end game, the microscope still makes sense.
If you remember, Light had Mikami hide the real Death Note, create a fake notebook and carry it around with him, writing names in it. He also had Mikami send Takada some real Death Note pages so that she would be the one doing the actual killing.
So if Mikami is verifying something about the fake DN, writing in it is pointless. If, on the other hand, it's the real one he's checking--Light told him not to use it.
He doesn't know Light's reasons and isn't likely to risk disobeying him if there are other ways to perform the verification.
Light reveals in his little mental monologue that he had Mikami check every day if the notebook had been tampered with. This is because Light knew that Near was onto Mikami and anticipated that the SPK might do something to it. When Mikami uses the microscope, he's doing it to verify the fact that the fake notebook HAS been tampered with (not the other way around), and lets Takada know, so that she can in turn notify Light. Thus Light is not surprised or worried when Near invites him to a showdown--he knows what is about to happen, and is ready. He lets Mikami know that he should come to the "showdown" as well, but with the REAL Death Note. So why doesn't Mikami just write a name on the questionable pages to see if they're real or not? Because he knows that the SPK is following him and that they might look at the notebook at any time. Light had him routinely leave it in the gym locker ON PURPOSE to lure the SPK into tampering with it. If he breaks the routine the SPK will be suspicious. If he writes names on the back pages instead of writing in order, and the SPK sees this, they will be suspicious. If he writes something and then uses an eraser, etc, they might still have ways to see this. It's too risky, especially if whatever Mikami wants to check can be easily seen by other means.