Filling the particles-bubbles by using threshold

[saule]

Hello, I'm currently analising bubble size change by using Image J. Aftersubstracting background and increasing contrast, I have tried to adjust thethreshold. However, I have to do a lot of work to clean, and erase all noiseand rubish around the bubbles. Is there's any way of doing an inversepicture or filling up the bubble inner part, so I could avoid cleaning step.

THank you very much,Sincerely,Daiva1_t000c3.jpg