The mirrors can be anywhere from a few inches to a foot apart. Then slip the mirror into the slit. With the reflecting surfaces facing each other, look through the hole into the space between the mirrors. You can also try placing either your finger or some other object between the mirrors. If you place an object between the mirrors, notice that there is a repetitious pattern in the orientation and spacing of the images.
Successive images alternate from front view to back view.
If the original object is closer to one mirror than to the other, the distance between successive images will alternate from close together to far apart - making the images seem to be grouped in pairs, with a front side always facing a front side, or a back side always facing a back side. You see images where the dashed lines come together. This light-ray diagram shows the rays that come from the front of an object and those that come from the back. As you look at the images formed in one of the mirrors on the diagram, notice that there is an alternation of front and back views, that the images appear to be grouped in pairs, and that a front side is always facing a front side and a back side is always facing a back side. This corresponds to what you actually observe in the mirrors.