The novel symmetrical carbocyanine dyes pro- (I) wherein R is an alkyl radical of l to 3 carbon atoms or a benzyl radical; R. is an alkyl radical of l to 4 carbon atoms; R is a hydrogen atom or an alkyl radical of l to 3 carbon atoms and X is the negative radical of an acid, e. g., halogen, such as Cl, Br and I; perchlorate, SCN, ptoluenesulfonate, methosulfate, ethosulfate, etc.