When setting up my Stellaris™ RNA FISH assay, why do I have to adhere my cells to a no. 1 coverslip?

For optimum clarity of signal from the individual transcripts, the sample needs to be as close as possible to the lens, and located in a neat uniform plane. The distance to detection includes the thickness of the glass to which the cells are adhered. When using a thicker glass surface, such as a no. 1.5 coverslip, the sensitivity is reduced and distinct Stellaris RNA FISH signals are more difficult to detect.