Purple Jars do not match with any known Idomatic expression. Rather, in these lines form the poem by Amy Lowell, the character uses metaphors and personification to describe a the condition of her mind. A strong theme of despair.
As if staring into the night from behind prison bars and waiting to be rescued, the lamenter paints a picture that reeks of hopelessness.
Here it means that when there is confusion and no hope. When there are purple jar of nights that is coming of night or darkness in your life or complete unsurety and you fall into it unaware of what is going to happen because you have no idea as to what is the solution of the problem.
Explanation:
These lines have been taken from the poem "Patience" by Amy Lowell.
