As it turns out, descriptions of this are already all over the internet:
- What are the lines visible between two cards held edge-to-edge?
- https://www.reddit.com/r/askscience/comments/4yolbi/when_i_hold_two_fingers_together_and_look_through/d6pp2ft/ (direct link to a very nice answer)
I don't think describing the procedure is necessary, just click on any of the links.
I tried it both with my fingers and with cards a lot and it seems to always work. I only tried artificial light sources, not sun light, for safety reasons.
However, isn't the light of incandescent light bulbs (I – among other kinds of light sources – tried it with several different ones of those) black body radiation which isn't coherent whatsoever? Why does it still work?
I'm not using any optical instruments (glasses, contact lenses, prisms, etc.), btw. Just a light source, my eyes, my fingers, and cards.