One player hides four atoms in a grid, and the other player has to guess their positions by discovering the outcome of firing rays into the grid.
Each player draws two 8 x 8 grids, labelled along the sides with the letters A to H and along the top and bottom with the numbers 1 to 8. Leave a gap between the labels and the grids to write in the result of each ray.
On the left-hand grid the player hides four atoms. The player will use the right-hand grid to try to deduce the positions of their opponent's atoms by firing rays into the black box:
The players take turns in firing a ray into the opponent's black box. To do this they specify one of the positions around the outside of the box, such as "D left" or "3 top". The opponent works out how the ray interacts with their hidden atoms, and then announces the result, which can be one of three options: a Hit, a Reflection, or a Detour.
A Hit is a ray that is adsorbed, and does not exit from the box. The player says "Hit" and the guesser labels it H.
This occurs if a ray hits an atom directly, as in the ray from E Right:
A Reflection is a ray that exits from the box at the same point that it enters. The player says "Reflection" and the guesser labels it R.
A Reflection occurs if a ray passes next to an atom on the edge of the box, like the one at C Right. A Reflection also occurs when two deflections occur in the same cell, as with the ray from F Left:
A Detour is a ray that emerges from the box at a different point from its entry point. The player names the exit point, and the guesser labels the entry and exit points with matching digits.
A Detour occurs if a ray passes close to an atom, in which case it will be deflected by 90 degrees. For example, the ray entering at D Right is deflected and exits at 4 Top:
A Detour can result from several deflections in sequence. Note that Detours are always symmetrical; the path is the same if the entry and exit points are interchanged.
A detour can also result from a ray that doesn't get adsorbed or reflected, and passes straight through the box. Again, the player names the exit point, and the guesser labels the entry and exit points with matching digits.
For example, ray 3 entering at 5 Bottom exits at 5 Top:
Note that this is indistinguishable from a detour resulting from three deflections in sequence:
At any stage in the game a player can try and guess the positions of the four atoms, by naming their coordinates on the grid; for example: B2, B8, E3, and G7.
The player is charged one point for every hit, one point for every reflection, two points for every detour, and five points for every atom guessed incorrectly. The player with the lowest score wins.