PHP MongoDB\Collection->find() exclude fields doesn't work?

Very recent install. Excluding fields in find() doesn’t seem to work.
I have tried every variation I can imagine of the following and still ‘Potnum’ is returned.

php > $a = $db->selectCollection("image")->find(array('Potnum' => 1), array('Potnum' => false, 'Dirpath' => true, 'Filename' => true))->toArray();
php > var_dump($a);
array(1) {
  [0]=>
  object(MongoDB\Model\BSONDocument)#276 (1) {
    ["storage":"ArrayObject":private]=>
    array(4) {
      ["_id"]=>
      object(MongoDB\BSON\ObjectId)#273 (1) {
        ["oid"]=>
        string(24) "5f3057ccd84aeb83c4872119"
      }
      ["Potnum"]=>
      int(1)
      ["Dirpath"]=>
      string(11) "a/burnished"
      ["Filename"]=>
      string(16) "watercarrier.png"
    }
  }
}

Answer found
The second argument to find() is an array of options.
The option limiting returned fields is called 'projection'.
The phrase in the original question should read:

$a = $db->selectCollection("image")->find(array('Potnum' => 1), array('projection' => array('Potnum' => false, 'Dirpath' => true, 'Filename' => true)))->toArray();