Historical aerial photographs are overlaid on a basemap (map or current aerial photo) in semi-transparency. You can change the period of aerial photographs by selecting from the pull-down menu on the upper left of the map, and the type of basemap from the icon on the upper right. Adjust the overlay transparency with the slider bar on the upper left.
Please note that if there is no aerial photo taken during the period, it will not be displayed and only the basemap will be displayed.