And, perhaps, in order completely to account for all the observed eccentricities of the light of Eros, the irregularity of form may have to be supplemented by certain assumptions as to the varying reflective capacity of different parts of the misshapen mass.
The invaluable Harvard photographs show that long before Eros was recognized as an asteroid its light variations had been automatically registered on the plates.

Some of the plates, Prof.E.C.Pickering says, had had an exposure of an hour or more, and, owing to its motion, Eros had formed a trail on each of these plates, which in some cases showed distinct variations in brightness.

Differences in the amount of variation at different times will largely depend upon the position of the earth with respect to the axis of rotation.
Another interesting deduction may be made from the changes that the light of Eros undergoes.

We have already remarked that one of the larger asteroids, and the one which appears to the eye as the most brilliant of all, Vesta, has been suspected of variability, but not so extensive as that of Eros.

Olbers, at the beginning of the last century, was of the opinion that Vesta's variations were due to its being not a globe but an angular mass.