Black Pool
Throughout the film, we watch as Marlowe is knocked unconscious time after time. Every time he's knocked out we see a black pool envelop him, and it becomes a symbol of him falling deep into the darkness of being unconscious. It also shows us that he begins to miss out on what's happening around him, thus making the story more and more mysterious.
Spider Webs
Marlowe is taken to a doctor who drugs him up in order to get information out of him. When he awakens he sees cobwebs all around him. The cobwebs are a symbol that the drug inside his veins has control of him and he isn't thinking nor seeing clearly.
Lamp
In the opening of the film, we see the white light of a bright bulb taking up the full screen just before we see Marlowe with his eyes covered with bandages. The bulb is a symbol that Marlowe is under the heat of the police as he is being interrogated by them for a slew of crimes and they believe he is at the center of it.
Doorways
Marlowe has been injected with a drug by the doctor on order by Amthor. We watch Marlowe walking through doorways as he falls into a hallucination. The doorways are a symbol that Marlowe is slipping further and further away from reality, and into a state of unconsciousness where Amthor can get information out of him.