Figure C is not treated as being especially important or interesting in the field of modern optical science.

Figure Y is the diagram which shows a virtual point light source. Insert K between P and your eye.
The shadow of K appears on the retinae, because we really see it through a virtual point light source(P).

Generally speaking, we must look at the light source itself so that we can see firsthand the shadow of a material or an object on our retinae. In the case of Fig.Y, however, the image of virtual point light source(P') is already made on the retinae. Thus, we are not conscious of the shadow of K even though without K is inserted. If you move the retinae straight in the direction of mark ← as in Figure Z, you will perceive the shadow of K(K') within Figure C.

Figure C is a virtual image of P and looks like a real image which exists in the distance of d4. It is peculiar that K' upright on our retinae is actually seen upside-down.

To simplify this explanation, I proposed that we move our retinae to the direction of mark ←. Needless to say, such a transfer is related to the curvature of lens mentioned in Chapter 1. When the curvature of the eye lens becomes thin, the image of P(P') is formed on spot farther than our retinae. As a result, the image made on our retinae is not real.

By seeing the virtual point light source(P) we can obtain the shadow of K(k') in Figure C on your retinae. At this moment we can find a very important fact.

When a thick material like K is inserted, it can not be penetrated by visual light so its shadow is made within Figure C. If a minute particle is inserted it can be penetrated by visible light.

Therefore the image of its inner structure appears within Figure C of our retinae. It is clear from this experiment that it is not electrons nor x-rays but visual light that penetrate the minute particles and create the image of the inner structure. Here, visual light means photon known as the finest particle today. We can call it light waves because we regard light as fluctuation.

The above discovery is so important that I would like to mention it one more time. When P penetrates a material, we can see its inner structure shown in Figure C which seems to exist at the distance of d4. Figure G is the front view showing us the inner structure of water.
Today x-rays and electron microscopes are widely used in examining the structure of material. These instruments print the image of a material on a film and a sensitive film when x-rays and electrons penetrate the material. This experiment shows that visible light produces the image of a material immediately on the retinae when light penetrates it. This image is quite different from the image made by x-rays or electrons.
To summarize we can see the inner structure of a material directly on the retinae by the light from virtual point light source. Hence, we can regard this method as a new instrument to search for the structure of a material just like x-rays and electron microscopes. We can't see the atomic world with x-rays or electron microscopes. But this new instrument can display the image of the atomic world on our retinae using a photon which is known to us as the smallest particle. Additionally, this method shows experimenters continuous motion of the atomic world and natural color of the structure of the material.

The former differs from the latter in respect that it shows us the atomic world which has never been revealed since the dawn of history. This method is named officially the Visible Light Penetration-scope.

"The End of the Theory on How to See an Atom:"