Actually the acceleration principle, in its original formulation, is 
not based on the expected rate of increase of income or output, but 
of demand for finite product. Aftalion, Clark and Bickerdicke are 
adamant on this. Also Carver, who argued in terms of values rather 
than quantities, related investment to the increase of consumption. 
Harrod reasoned in terms of expected increase consumption in the 1936 
book on The trade cycle, and in the 1939 EJ article switched to 
income (no longer expected, to Keynes's disappointment: see their 
August 1938 correspondence beginning here: 
http://economia.unipv.it/harrod/edition/editionstuff/rfh.35e.htm ).

Daniele Besomi