24 



THE 6O-FOOT TOWER TELESCOPE. 



But the Snow telescope was not free from limi- 

 tations. During long exposures its mirrors were 

 seriously distorted by the sun's heat and the effect 

 of heated air from the earth was plainly shown by 

 a blurring of the solar image. To obviate or reduce 



FIG. 17. 6o-foot Tower Telescope. 



these difficulties the vertical or tower telescope 

 was devised, and constructed in an inexpensive 

 form. After reflection from two plane mirrors at 

 the summit, the sun's rays pass through a 1 2-inch 

 objective of 60 feet focal length, which forms an 

 image of the sun on the slit of the spectrograph in 

 the observing-room at the foot of the tower. The 



