I am currently studying the textbook Modern Optical Engineering, fourth edition, by Warren Smith. The textbook presents the following images and explanation:
In Fig. 1.9 the action of a concave lens is sketched. In this case the lens is thicker at the edge and thus retards the wave front more at the edge than at the center and increases the divergence. After passing through the lens, the wave front appears to have originated from the neighborhood of point $P'$, which is the image of point $P$ formed by the lens. In this case, however, it would be futile to place a screen at $P'$ and expect to find a concentration of light; all that would be observed would be the general illumination produced by the light emanating from $P$. This type of image is called a virtual image to distinguish it from the type of image diagramed in Fig. 1.8, which is called a real image. Thus a virtual image may be observed directly or may serve as a source to be reimaged by a subsequent lens system, but it cannot be produced on a screen. The terms “real” and “virtual” also may be applied to rays, where “virtual” applies to the extended part of a real ray.
It is this part that confuses me:
Thus a virtual image may be observed directly or may serve as a source to be reimaged by a subsequent lens system, but it cannot be produced on a screen.
I thought the entire point was that the virtual image cannot be observed directly (or, actually, observed at all). So I'm wondering what the author means by this part?
I would greatly appreciate it if people would please take the time to clarify this.