FROM an experimental point of view, Einstein's general theory of relativity is at present in an ambiguous position. It is well known that there are three conceivable tests between its conclusions and those of the traditional ideas which it attempts to displace. With regard to the first of these—the movement of the perihelion position of Mercury—the success of the theory is decidedly impressive; all the more so, perhaps, because the result was stumbled upon, as it were, involuntarily. In seeking first the gravitational field of the sun, Einstein found the true orbit of Mercury added unto him. On the other hand, the predicted displacement of the solar spectrum lines certainly conjures up a serious obstacle. The evidence, it is true, is contradictory, but, such as it is, it seems to show a balance against the existence of the displacement. The extreme difficulty and complexity of the experimental work must, nevertheless, be borne in mind. Perhaps it is scarcely possible, in the present state of our knowledge and experimental equipment, to obtain a definite solution of the problem. The third test—concerning the deflection of light in a gravitational field—accordingly becomes of very considerable importance, and to many minds constitutes the deciding factor in their judgment of the theory.