checkbox - multiple check box error in rails with the help of hash -

in @eyecolor contain hash

{"brown"=>"brown", "black"=>"black", "blue"=>"blue", "green"=>"green"}. 

now want show multiple check box in rails, , have implement code this

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

and want check black

but didn't show me multiple checkbox, display me 1 check box.