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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. 



Fig. G. 

 Great Nebula in Okion. 



Photographed with the forty-inch Yerkes telescope (Ritchey). 



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 ita 

 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 nebulse. Less than half a century has passed 

 since the true nature of a gaseous nebula was determined. In his ex-* 

 tensive observations of astronomical phenomena Sir William Herschel 

 examined a great number of star clusters similar to that shown in 



