As you all (nearlyish) know, one should never place a sample on a thick glass slide and add a coverslip. Instead, the sample should be placed on the coverslip then covered with a thick glass slide. And the coverslip should be 170 um thick (also labelled thickness #1.5).
Why is that? Because the coverslip is part of the design of the objective and all objectives from all manufacturers are designed to image through 170 um glass and assuming that the sample is directly in contact with the coverslip.
What about superfrost slides that one uses to make sure tissue sections don’t float away during antigen retrieval? No worry! You can make your own superfrost coverslips. It is cheap and you can prepare tons at the same time. Here is the protocol.
Not convinced? You will only see the difference when you compare side by side! The images of your tissue will be much sharper if the sample is on the coverslip because when you put your sample on the slide, both the thick glass and the mounting medium end up between the objective and the sample. The objective is not designed for this. 😀