The Nexus 7 FHD (2013 model) has both a front facing camera and a rear camera. The rear camera is about as good as your average smartphone for close up focussing so it probably won't do as a microscope. For info its a 5 mega-pixel device.
I've tested my son's nexus with a USB camera and an USB OTG adaptor (allows you to plug USB devices into the Nexus USB port) but it didn't detect it so I guess the idea of a USB based microscope camera is out as well.