You are seeing a 65-year-old male in A&E who has presented with left eye pain. This patient reports of sudden onset severe pain, a headache and nausea. He has vomited three times since the pain has come on. He is also complaining that he is seeing halos around objects in his vision. On examination you note that the eye is red. You need to admit this patient urgently as you suspect acute angle closure glaucoma. 20 minutes later the ambulance still hasn’t arrived and you need to start emergency treatment in primary care. What is the most immediate step in management?