Filling the particles-bubbles by using threshold

[saule]

Hello,

I'm currently analising bubble size change by using Image J. After substracting background and increasing contrast, I have tried to adjust the threshold. However, I have to do a lot of work to clean, and erase all noise and rubish around the bubbles. Is there's any way of doing an inverse picture or filling up the bubble inner part, so I could avoid cleaning step.1_t000c3.jpg
THank you very much,
Sincerely,
Daiva