In a recent paper Van Vleck gave an approximate method of estimating the effective potential due to Majorana exchange forces, which acts on a proton or neutron when they collide with a nucleus. The effective potential obtained in this manner varies with velocity. Calculations reported on here show that the effect of this variation with velocity on the shape of the excitation curve of lithium under proton bombardment is small and is of no practical importance. The position of stationary proton levels which are consistent with the excitation curve is affected seriously. Agreement of calculations with the mass defect of ${\mathrm{Be}}^8$ is materially improved through the use of the exchange potential. With a radius 0.35 × ${10}^{-12\mathrm{}}$ cm the empirical mass defect of ${\mathrm{Be}}^8$ is accounted for perfectly by the calculation which simultaneously fits the excitation curve for the 8-cm particles.

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