Microscopes left exposed to cigarette smoke, cooking vapours, incense and other household fumes rapidly mist over with a deposit of particles and droplets which condense and settle out of the atmosphere.

In these micrographs, the top lens of an anti-reflection coated eyepiece has been removed and placed under a microscope set up for vertical illumination brightfield. The resulting images, when closely examined, are an interesting mix of real objects and optical artefacts -- the latter being mostly interference fringes, more commonly known as Newton's rings.

Clicking on the central object of the above picture will produce an enlargement which is a good illustration of a "mixed" image. The interference fringes have formed a pattern similar to the contour lines of a map showing altitude, only here the lines represent the depth of the film of oil which has flowed out of the central particle and formed a meniscus around each of the numerous smaller surrounding particles.
The clear zones formed around other particles are no doubt caused by surface tension effects far too mysterious to explain.

The deep blue of the background is due to the near-perpendicular incidence of the illuminating rays in this form of lighting allowing the cancellation of all but the blue rays of the visible spectrum for which the coating (magnesium flouride) is designed. It has provided a contrast which enhances the visibility of the interference effects.
The objective used was a planchromat x16/0.35NA Zeiss Epiplan HD, and the images were recorded on Fuji Velvia transparency film.

The four pictures below are of other interesting dirt particles, all at a magnification of around x600. A soft tissue moistened with cigarette lighter fluid is an effective way of removing these greasy deposits from lens surfaces.