Smectics A exhibit two types of defects: 1.The focal conics, involving large-scale curvature deformations of the

layers. These extended defects are distinctly visible using a polarizing microscope and played a historical role in the discovery of liquid crystals. Indeed, it is by studying their topology that G. Friedel was able in 1922 to predict the lamellar nature of smectics, later confirmed by X-ray analysis [1];

2.The dislocations, locally breaking the translational order and involving local variations of the layer thickness. These defects are not always visible using polarizing microscopy, but can be revealed by other techniques, for instance by freeze-fracture and electron microscopy. F.C. Frank was the first to direct the physicists’ attention to their existence in smectic phases in 1958 [2].