multiple check box error in rails with the help of hash

In @eyecolor contain hash

{"Brown"=>"Brown", "Black"=>"Black", "Blue"=>"Blue", "Green"=>"Green"}.

Now i want to show multiple check box in rails, and i have implement this code for this

<%= check_boxes_tag "eyecolor[]", options_for_select(@eyecolor, params['eyecolor']),{:multiple => true,:size=>5} %>  

and i want to check Black

but it didn't show me multiple checkbox, It display me only one check box.