Page:Popular Science Monthly Volume 60.djvu/308

300 moving away from the point of intersection of the cross-hairs, he instantly brings it back by means of one or both of the screws. As the plate moves with the eye-piece it is evident that this method furnishes a means of keeping the star images exactly at the same point on the plate throughout the entire exposure. With such apparatus data are gathered for the study of stellar development.

It is easier to trace the successive steps in the development of a star after it has been formed than it is to account for its origin. But all the evidence that has been accumulated up to the present time tends to show that stars are condensed out of the cloudlike masses which we know as nebulæ. Less than half a century has passed since the true nature of a gaseous nebula was determined. In his extensive observations of astronomical phenomena Sir William Herschel examined a great number of star clusters similar to that shown in