The poet has resolved to travel to Innisfree, according to the meaning of these verses. He expected it to light up and emit the sparkle at midnight. There would be a purple light all around the earth in the late afternoon. But when night falls, everything becomes dark except for the stars which shine with bright lights. This is because stars are far away from the earth. So the poet thinks that at midnight when stars are at their highest point in the sky, they will give off the most light. Stars are also very large. So even though there are a lot of them in the sky, they can't be seen as a whole until you use a big telescope or binoculars. If you were to look through a small hole in a piece of paper, you might be able to see one star but not all of them together. In conclusion, midnight is all a glimmer because night falls over the earth at midnight when stars are at their highest point in the sky.