Nikola, I have very little idea what you're asking, but I think the answer is "Sort of, but not really, and I'm not quite sure."
The chromatic aberration you usually see is "transverse" (TCA). It amounts to having slightly different image sizes for the RGB components. TCA causes color fringing that varies from none in the center to large on the edges, and it's the same in all directions (radially symmetric).
In these pictures, I see three kinds of color fringes.
The first kind is at the edge of the circular field. It looks yellow on the left, not much of anything on the right. That's probably due to having the camera slightly off-center with respect to the eyepiece.
The second kind is at the edge of the wavy black line near the left side of the pictures. It looks cyan on the right side of the line, not much of anything on the left. That dark line is the edge of a water drop, and I suspect the color fringe is a prism effect from the sloping edge of the drop.
The third kind, which is what I suspect you're really taking about, consists of narrow fringes around the edges of the protozoa themselves. They look sort of like TCA in that they tend to be red/magenta on one side of the critter and cyan/green on the other. But the fringes don't get bigger toward the edges of the field and there's nowhere in the image that they don't exist.
Those fringes had me stumped for a long time. If they were formed by a prism effect at the edge of each critter, then I would expect them to be symmetric with respect to that edge, and they're not. TCA doesn't seem right, as discussed above. I've played with every adjustment on the scope, and I couldn't find anything that changes the fringes very much. I've tried reducing them with PTLens (which works well with most TCA), but whatever makes them better in one place makes them worse in another. So what's going on?!
It turns out (I'm pretty sure) that these fringes are primarily due to
longitudinal color aberration (LCA). In LCA, the RGB images are about the same size, but they're focused in different planes. That's a problem when looking at protozoa, since focusing in different planes means that the RGB components are actually showing different structures. No wonder the color fringes don't make great sense!
The reason I say "I'm pretty sure" is from studying these images in Photoshop. Using the Channels tab in the Layers window, it's easy to look at Red, Green, or Blue separately or in any combination. Switching between Red and Green gives an impression very much like tweaking the focus knob. Some structures get more blurred, while others get more clear. In some places (for example the wide part of the paramecium), switching between Red and Green components appears to change the relative positions of various organelles. This suggests that the lens also has a slightly different center of perspective (entrance pupil location) for Red and Green.
The focus shift between Blue and Green is much less than for Red and Green. Relative intensities change for different organelles, but location and focus stay pretty much the same.
I presume that these fringes would go away if I were to use a much higher quality objective, say an apochromat or fluorite instead of my current inexpensive achromat (see discussion
here). But that's not going to happen for a while -- the interest-to-cost ratio just isn't high enough to justify it.
As usual, thanks for the question. I had seen these color fringes before, but never investigated carefully enough to really understand them. I feel a bit less confused now...
--Rik
)
