The blue line artifact (IV)
A four episodes' lecture about microscopic deception

Let's continue with our deep dive into - what we think is - microscopic reality. Please remember the cause for this discussion, a photograph as already shown in our previous issues (fig. 1). At first glance it looks like a primitively tampered image - with someone drawing a blue line around the egg! But again - believe it or not - this is an actual image, directly out of the microscope camera.

Fig. 1: The cause for our discussion - a tardigrade egg photomicrograph, with a strange blue contour line!

Admittedly, we took you to a misleading sideline - about the color effects encountered when looking at diatom shells.

We simply wanted to provide more time for you reasoning and discussion. In the meantime we received a full direct explanation hit by one of our readers which is in accordance with our own simulation experiments and conclusion.

Simulation #1

As reported in our February issue, the tardigrade eggs are displaying those blue lines only in intact, fully globular state. Furthermore we can assume that the egg shell should be made up of an optically denser material than water. As a consequence we tried a simulation with a glass hemisphere inspected against a perfectly gray sky. Similar as in the case of the egg the glass material is optically denser than its air environment.

Fig. 2: Simulation of the conversion of a neutrally-gray, cloudy sky to an intensively blue color. For improved handling the glass sphere was glued to a glass pane. When observing the gray sky tangentially through the semisphere and turning it about its vertical axis a typical sequence of rainbow colors (in form of concentric rings) will appear, one after another. In the most extreme position, the one shown here in the photograph, only blue light is making its entry into the semisphere, followed by a final black (complete darkness). As you can see the semisphere is working similar to a prism in this setup, separating daylight into its spectral components.

The conclusion from this experimental setup is that a spherical surface geometry can function similar to a prism when light is entering at an extreme tangential angle, separating the gray sky light into its various wavelength proportions. A combination of dispersion (to various color rings) and filtering (to pure blue) is the obvious result.

Simulation #2

As a second step we are going to reduce the dimensions of our object in order to approximate the tinyness of a much smaller object. As we didn't come across suitable glass spheres we resorted to a thin glass cannula (i.e. a hollow glass needle). Such a cannula can be fixed to an object slide by means of sticky tape an then be observed under a microscope. At low magnification, under the dissecting microscope, the cannula looks just like what one might expect: simply transparent with no color effect at all:

Fig. 3: The experimental setup with a hollow glass needle, as seen under a dissecting microscope, at low magnification. The hollow glass needle has an outer diameter of 180 µm (0,18 mm). It is fixed to a normal microscopic slide glass by means of transparent sticky tape (as can be seen on the right side of the image). The glass definitely doesn't provide any blue contour line here - none at all!

But watch:

Fig. 4: The same setup as in fig. 3, but at slightly higher (medium) magnification. It took us some time to fine-adjust the microscope substage condenser in combination with the iris diaphragm, in order to obtain the two blue contour lines as shown here in the photograph. But suddenly they jumped into place - disctinct and with a beautifully saturated blue color. That's it!

Fig. 5: Same setup as in fig. 4 but this time at strong magnification. Please note that the blue lines appear to be not exactly at the glass edges but slighty displaced to the inner volume of the glass. This is in full accordance with the bizarre blue line placement in fig. 1, making the effect even more mysterious.