even the dark side of the fainter body is 16 times as bright as our Sun. And the chief part of this knowledge comes, not from using a very large telescope, but from putting a sensitive apparatus in place of the eye end.

There is one more thing I must tell you in concluding this lecture. I must answer the question about Professor Hale's tower how it is kept from shaking. We could keep a reed from being shaken by the wind if we enclosed it in a tube: the wind would blow on the tube and perhaps shake it, but could not get at the reed. Fig. 36.—Mr. Stebbins's Representation of Algol. Professor Hale's idea is to build a tower of rods and surround each rod by a tube, joining all the tubes together to form an outer tower or casing, on which the wind may blow and which perhaps may shake, but which keeps the wind off the inner tower altogether. The coelostat is of course attached to the inner tower, so that it may not be shaken. "The proof of the pudding is in the eating," and such an idea can only be justified by trying it. It has been tried and found completely successful. Professor Hale finds that even in high winds the image of the Sun in his big tower telescope does not shake at all!