  The differential cross section in the transverse momentum $Q$ and a total
cross section of $(31\pm4)$ mb for the coherent dissociation of a 3-A-GeV/$c$
$^7$Li nucleus through the $^3$H$+^4$He channel have been measured on emulsion
nuclei. The observed $Q$ dependence of the cross section is explained by the
predominant supposition of the nuclear diffraction patterns on light (C, N, O)
and heavy (Br, Ag) emulsion nuclei. The contributions to the cross section from
nuclear diffraction ($Q\le400$ MeV/$c$) and Coulomb $(Q\le50$ MeV/$c$)
dissociations are calculated to be 40.7 and 4 mb, respectively.
