Sherlock sat at the table in the kitchen of 221B. It was just after lunch and he was in the middle of an experiment about mould growing on the top of old cups of tea and coffee to establish how long a flat had been unoccupied. It was part of the proof he needed to show that someone could have committed murder, before they emigrated to Australia and had settled there for 2 months. He pulled himself away from the microscope and took a deep breath. His head was throbbing. It wasn't just throbbing, it was pounding. One of those headaches where you just wanted to lie down and close your eyes.

He picked up his drink and walked over to the sofa. He grabbed a cushion, lay down slowly and shut his eyes. He needed a break from looking at text and into microscopes, which was not like him. Sherlock used to be able to work for hours without a break, but recently something was amiss. As he lay and closed his eyes, his thoughts wandered into his mind palace.

It wasn't the first time in recent days and indeed recent months that a morning spent looking into a microscope, writing detailed notes or reading a book had resulted in such a headache. Mrs Hudson had popped up for her usual mid morning cup of tea and a chat so he couldn't have been working for more than 2 hours maximum. His thoughts focussed. Headaches after looking in detail at a microscope, or reading. Even Sherlock Holmes himself could work out what the problem was. He just was not prepared to admit it. He would rather continue to suffer and struggle than to admit a part of his body was failing. Sherlock Holmes did not fail.

He must have dozed off for a couple of hours, because he suddenly became aware that it was getting dark outside. It was the middle of winter and so that meant it was around 4pm. He knew that Molly would be in the lab at St Barts. It was getting cold in the flat. He couldn't see the point in putting the fire or the heating on just for himself, so he scooped up his notes and some petri dishes containing his experiments, and put his coat on. Some company in the lab would be very welcome, and he was sure that although Molly wouldn't admit it, she'd welcome some company too.

Sherlock had shaken off most of the headache by the time he got to St Barts. He grabbed a strong coffee on the way into the lab, hoping futilely that it would take the rest of the headache away. 'Caffeine, I just need caffeine' he told himself over and over.

When he entered the lab it was empty but Molly's bag and her phone were there so she was around somewhere. He quickly scanned the white board - she was up in the morgue carring out a post mortem. 47 year old female, stab wound to the left hand side of the chest, bruising around the left wrist and trauma to the right knee. He picked up a pen and wrote next to it. 'Domestic. If husband is left handed, it was him' before sitting himself down at his favourite microscope.

He set about sorting out the microscope to his liking, changing the height and angle and refocusing it. 'How anyone can see through that is beyond me' he thought to himself.

He lost himself in his work, carefully noting everything he found. There was access to so much more equipment and chemicals in the lab than ever he had at home. He was able to undertake much more complex chemical analysis using the equipment, and to find the results much more quickly. It was one of those rare occasions when he wondered if he might have suited lab work - before he came to his senses and thought in pure horror about the idea of fixed hours of work, having to turn up on time every day. It filled him with fear!

He was half way through his experiment when his headache returned with vengeance. He buried his head in his hands to relieve the pain, before going on a hunt around the lab for some pain killers. He had just tracked down some ibuprofen, when he heard the lab door click open and Molly walked in.

"Hi Sherlock, what are you doing here? Oh, and what have you got there?"

"Just some ibuprofen, and before you ask, I'm clean otherwise. OK?"

"OK, I believe you. Got a headache? Looks like a bad one judging by how your face is all screwed up. Didn't know you ever got them!"

"Yes, thank you Molly." he said, popping the pills in his mouth and swallowing them down with a glass of water.

She wandered over to the white board where she saw what he had written and laughed. "Yes, I think you're right. Does look like a case of domestic violence to me."

How typical of Sherlock to not have even seen the body but to be able to work out straight away what had happened. She watched him with interest, as he returned to his work place and started to look down the microscope. But something was slightly different this time. She'd not seen him at work for quite some time due to his time away following 'the fall' and she watched in interest as he looked down the microscope, pulled his head away, shook it slightly and then returned to look down it again.

The Sherlock she knew was able to spend hours staring into a microscope while writing notes at the same time. He never looked at his piece of paper, but his notes were still completely legible and clear. But today it was different. He appeared to only be able to look into the microscope for a matter of seconds before having to look away.

"Sherlock, I don't mean to pry, but are you OK? You seem rather agitated. Is your head really bothering you?"

"I'm fine Molly, thanks. But can you come and look at something for me. A mans alibi depends upon this and I need to know I've got it right."

"Of course" said Molly wandering over to his work place amused that he was asking for her confirmation on something. She felt honoured he trusted her enough to ask her to share in his experiments, which were always complex and very interesting. A branch of chemistry and forensic science she never got time to explore herself, so she relished these opportunities to work with him.

She peered into the microscope, and had to pull straight back. "What setting have you got this on? Can you seriously see through this? I can't see a thing."

"I adjusted it a little. It was all out of focus. I still can't quite see clearly which is why I need you to confirm it for me, but its better than it was."

"Sherlock, please answer me seriously" she said looking him straight in the eye "Can you not see this? And you've got headaches? This is all blurred." She paused and thought for a while before the penny dropped. "I think we need to get you to an optician first thing in the morning"

"I'm fine. Just tired and been spending too much time working on this, that's all" he said snappily, starting to pack his things away.

Molly grabbed Sherlock's arm and stopped him. "Sherlock, please. I'm not having a go at you. But if you seriously have to have the microscope this far out of focus to partly see, and you're suffering from headaches which you never have in all the time I've known you, you need to get this looked at. If your sight isn't as it was, it'll only get worse, not better. Trust me, I know. I went through the same thing about 5 years ago. As soon as I got my glasses and subsequently my contact lenses I was so much better. I'll come with you if you like? A friend of mine is an optician. He'll sort you out. He's been sorting my eyes for years."

Sherlock sat down and looked at Molly. He took a deep breath and nodded his head slowly, taking care not to make any actions that were too violent for fear of more pain. "It's been slowly getting worse for some time. It started very suddenly when I was away in Eastern Europe. Took a particularly bad beating to the head during one reconnaissance trip to track down some of Moriarty's gang, and I noticed the difference straight away as the deterioration was so sudden. It's been slowly getting worse over time."

"If it's related to a blow on the head, then yes it can happen quite quickly as you describe. I often find that with people I'm doing PM on. If they've taken a particularly bad blow to the head you can see evidence of eye sight problems, even in death. It's better to admit it, trust me. You'll find life so much easier if you do. I hadn't appreciated what I couldn't see until I got my glasses. I'll phone Paul in the morning and get you an appointment and text it over to you." She bent over and gave him a kiss on the cheek. "Now, go and get yourself home if you've finished here. You need some sleep to get rid of that headache."

"Got any morphine I can have?" asked Sherlock grinning

"No I haven't" Molly replied straight back. "Now get out and get home!"