Qualitative illustration of lensing along a primary curve. Left: The lensed images of a disk shaped color wheel source. The dashed curve shows the primary curve ξ(t) parameterized by t. The values of t along the primary curve are shown as solid ticks. Top-right: Values of the primary eigenvalue λ₁(t) along the primary curve. The places where λ₁(t) hits 0 corresponds to the critical curve crossings where the magnification is infinite. Bottom-right: Unlensed image of the color wheel source. The dashed curve shows the set of points that we get when we map the primary curve onto the source plane.