Microscopes have weird size lenses with weird focal planes so prime focus with them using a DSLR without a relay lens doesn't always work. Good quality relay lenses are not cheap either but with one like this you will be able to use your DSLR for photography through the microscope. Without it you may not reach focus with some objectives.
http://www.ebay.com.au/itm/Canon-SLR...item5d4abf0fcb