NOTE : "Generally speaking, is virtual anything which exists only as a potential and not as an act; but this can be understood in both ways". Lalande, dictionnaire technique et critique de la philosophie, PUF. In the virtual article, in which Mr Winter reminds us of the principle of virtual speeds: "When a system of material points linked together in some way is balanced, the summ of the virtual works of all the forces directly applicated to the system, is nil for every system of virtual shifting compatible with the links". The 1947 edition doesn't judge necessary to consecrate a special mention to the expression of "virtual picture", which use is reminded us by René Berthelot.