Brightly birefringent, needle-shaped, urate crystals aspirated from a gouty tophus can be seen in this image taken with a polarizing microscope and a red compensator filter. The negatively birefringent crystals appear yellow when they lie parallel to the optical axis of the compensator and blue when perpendicular to it.
By permission of the publisher. From Myers S: Atlas of Rheumatology . Edited by G Hunder. Philadelphia, Current Medicine, 2005.