In Secret of the Silver Earring, how many items do I have to collect before I can go see Watson?

Are you in the first part of the case? If so, download the manual, it walks you through that in detail. Once you find all the evidence you need/talk to everyone, it will automatically bring Holmes & Watson together.

As someone who finished this years ago, then played it again now, I can tell you this (and anyone else who comes here looking for answers): you WILL need a walkthrough. Have one ready but don't read it until you think you have finished the current level but you can't proceed yet.

More general advice regarding the first part: some items you need to find are a few pixels big; if you think you see a mistake in the floor coloring, it probably isn't. Sometimes, when you find an object, there are others near it, but the washed out colors hide them. This also applies to objects that are somewhat large but their colors blend so well with the background you can't see them. Pass the cursor around everything and see if it changes.

More specific advice regarding the first part: You can't meet Watson until you have found absolutely everything BUT you can switch play to Watson by clicking EXIT in your map. You can only do this once. When Watson talks with everyone -even if he doesn't ask them everything he can- play will switch to Holmes. Once Holmes is 100% finished inside, he will automatically meet Watson outside. Once Holmes is 100% finished outside, play proceeds (not to give spoilers). Once it's time to leave for good, use your map.

If you think you already did everything you were supposed to do, make sure you examine all items at least once, and read about them in your notebook by right-clicking. BTW, your notebook bar sometimes obscures important things, so you should keep it off by default.

The game's good (WITH A WALKTHROUGH), and although I realize it's about observation, it should be more helpful in letting you know if you are finished with a room or not.

Finally, make sure you save every now and then. I won't give spoilers, but there are times in the game when you WILL need to load a previous save. A new save whenever you change locations (not rooms, locations) should be enough.

Can anyone send me a gamesave that was saved shortly after the bucket/ pond trial. I'm simply not quick enough to put the fire out. Been trying for ages. Many thanks.