Prof. Bohlin's result is rather unexpected, and will require confirmation before it can be accepted. But it will be interesting to inquire what this parallax implies as to the real dimensions and probable mass of this vast nebula. The extreme length of the nebula may be taken to represent its diameter considered as circular. For, although a circle seen obliquely is always foreshortened into an ellipse, still the longer axis of the ellipse will always represent the real diameter of the circle. This may be seen by holding a penny at various angles to the eye. Now, Dr. Roberts found that the apparent length of the Andromeda nebula is 2⅓ degrees, or 8400 seconds of arc. The diameter in seconds divided by the parallax will give the real diameter of the nebula in terms of the sun's distance from the earth taken as unity. Now, 8400 divided by 0"·17 gives nearly 50,000, that is, the real diameter of the Andromeda nebula would be--on Bohlin's parallax--nearly 50,000 times the sun's distance from the earth. As light takes about 500 seconds to come from the sun to the earth, the above figures imply that light would take about 290 days, or over 9 months to cross the diameter of this vast nebula. Elementary geometrical considerations will show that if the Andromeda nebula lies at a greater distance from the earth than that indicated by Bohlin's parallax, its real diameter, and therefore its volume and mass, will be greater. If, therefore, we assume the parallax found by Bohlin, we shall probably find a _minimum_ value for the size and mass of this marvellous object.