How do I determine the best threshold for this image?

I want to be able to create a binary matrix of this image. The particles will be 1 and the background and noise 0. How do I go about picking the best threshold value and is there an automatic way to do it for future images?

[Walter Roberson]

We've gone through this before. You have been unable to provide an explanation of which parts are the particles and which parts are background or noise and how to tell the difference.

I have not yet seen any evidence that refutes my hypothesis that you are taking images of something in fluid, seeing responses from different layers, seeing accidental geometric correlations, and making distinctions that are hard to justify.