In the Introduction to Part I we discussed the orbital structure of the C > C and >C = O double bonds and the difference between these structures in their excited states. With so much spectroscopic evidence upon these isolated double bonds, and at least a provisional theoretical description of each of them, it seemed natural to investigate the effect of double bonds upon each other in conjugated systems such as > C = C - C = 0. These systems occupy an important place in chemical theories of valency. Another reason for investigating such molecules was that measurements under low dispersion by Luthy upon the simplest conjugated aldehyde CH 2 = CH. C = O | H showed a spectrum which gave promise of unusual features. This promise was more than borne out by observations under high dispersion, and in fact the spectrum of acrolein reveals a tine structure which so far as we know is unique in molecular spectra at present recorded.