Best method to remove dust from image

[cejones1]

I have some dust on a microscope lens that I cannot remove, but I can take a background image of it as well as an image of my specimen. The dust shows up as a few darker spots in the specimen image as well as the blank background image I take.

I tried to simply minus the blank image from the specimen image, but it seems to inverse the image.

i have tried a number of the mathematically methods to remove the dust but I haven't been able to figure out one that works best. Has anyone else done something similar or have some advice for me?

Thank you for any help,

[dlemstra]

Continued here: ImageMagick/ImageMagick#2440