Elderflower or Sambucus nigra.
Collect the flower in early summer (May and June flowering). Cover the freshly picked flower heads with boiling water and leave to cool. Take the solution and use a dropper or cotton bud to apply to your eyes. I used this 2 days ago, it works and is an old and I thought well known remedy. Also for cats and dogs with conjunctivitis. Putting vinegar in your eyes just sounds silly. Sambus flowers are very antiseptic. The same thing with a cooled solution of sugar and water poured over the flowers and left for 48 hours makes a wonderful cordial. This eventually will form a alchoholic drink if left to ferment as the flowers supply their own favourable yeasts.