I believe more of a mysterious tone but im not for sure
Answer:
compound
Explanation:
the first clause is linked with the second one using the coordinator "but", which form a compound sentence.
It is dark because Barnardo has trouble seeing the other guard.
This scene takes place at midnight and it is so dark Barnardo cannot see the other guard. Barnardo shows up for his shift, but he can't clearly make out who Francisco is. Francisco has the same problem because instead of answering Barnardo he tells him to identify himself first. They get it figured out and Barnardo begins his shift.
