The nuclei of elodea cells are nearly invisible, but can be viewed via microscopy if the cell is stained. Scientists use a variety of stains to highlight cells and their structures, in order to properly study them under magnification.
In order to view the nuclei of an elodea cell, the observer would first need to stain the cell according to whether they would like to stain it living or dead. If the observer wishes to see the cells behavior, he would treat it with a dye solution known to not harm the species, and mount it to a microscope slide. If he wished to observe the cell in a static state, he would perform fixation, via heat or otherwise, to kill the cell. Then the cell would be stained and mounted to a slide.