Select the eye symbol and set the fill to none (click the crosshatched box to the left of the screen palette with the left mouse button). Set the outline weight to 6 points and the outline color as shown.

Make a duplicate (Ctrl - K), change the outline to 3 points and change the outline color as shown. Make another duplicate, change the outline weight and outline color as shown.

NOTE: I have moved the three duplicates apart to show the differences. Yours should be centered one on top the other.

Using the Zoom Tool (the magnifying glass icon), drag a rectangle around a portion of the symbol so that the three lines are clearly visible.

Select the Blend Tool. Click on the widest outline and drag to the next widest line. Click the Blend Tool on the second widest line and drag to the center line.

The results of this two-part blend should look like the section on the right.

NOTE: Because of the size of the image, this blend is not really necessary unless you are going to commercially print your image. But it is helpful to know the technique.