What are these markings for?


I picked up this interesting gadget from an army surplus shop. I've counted a total of 10 different tools, including a mirror. However I am slightly perplexed by the slit and the circular dotted pattern which is cut into the mirror - does anyone know what these could be for? Are they somehow necessary for the thermometer and hygrometer which are on the other side? This seems unlikely to me as the instrument would usually be closed so the holes in the mirror would be covered by the back half of the thing. The mirror is glued into position, so it cannot be taken out for use.

https://www.dropbox.com/s/dh95clpvrwxr2q3/Photo 26-02-2018, 18 20 04.jpg?dl=0
https://www.dropbox.com/s/6n2ojscat0jaser/Photo 26-02-2018, 18 19 54.jpg?dl=0