The narrow foot-plateau of British East Africa broadens out to the south of Bagamoyo to a width of over loo m.
The anterior segment broadens and becomes umbrella-shaped; it has a powerful row of cilia round the rim and smaller cilia on the general surface.
Here it broadens into Lake Ibrahim (Kioga) (in reality a vast backwater of the Nile discovered by Colonel Chaille Long in 1874), and continues navigable (save for sudd obstacles at times) right through Lake Ibrahim and thence northwards for loo m.
South of the isthmus aforesaid, North Island rapidly broadens out.
The cadmium line having a wave-length of 2288 A broadens by pressure equally in both directions, but if mercury be added the broadening is more marked on the less refrangible side.