One of the questions which will be of interest to those who wish to use the information presented in this series of reports, and which may be of interest to others as well, is the effect of the spatial extent of the source on the neutron transmission of an air slot in water. The method which has been chosen to investigate this question takes advantage of the fact that the source plate is continuously movable. The region beneath the slot is scanned with the leading edge of the forty inch square source plate to determine the relative importance of the various parts of the source in supplying the transmitted neutrons. The change in the transmitted flux when the source is moved from position A to position B is a measure of the flow originating in the regions which are not common to both position A and position B. This report presents the data which have been obtained on the source size effect and a discussion of some measurements which we expect to make in the near future to aid in answering the questions which these data have raised.