STELLAR EVOLUTION. 297 



tube, 64 feet in length, is supported at its middle point by the declina- 

 tion axis, which in its turn is carried by the polar axis, adjusted to 

 accurate parallelism with the axis of the earth. By means of driving 

 mechanism in the upper section of the iron column the whole instru- 



FlG. 4. 

 Star Cluster Messier 11. 



Large scale photograph taken with the forty-inch Yerkes telescope (Ritchey). 



ment is turned about this polar axis at such a rate that it would com- 

 plete one revolution in twenty-four hours. Although the moving parts 

 weigh over twenty tons the telescope can be directed to any part of the 

 sky by hand, but this operation is much facilitated by the use of electric 



