D. Todd — Open- Air Telescope. 



Fig. •'). 



Fig. 5. 40-inch Telescope of the Yerkes Observatory. (Glass by JIantois. objective 

 by Alvan Clark & Sons, of Cambridgeport. mounting by Warner & Swasey. of Cleve- 

 land. The tube is about 65 feet long: and. although the moving parts of this great 

 telescope weigh over 15 tons, the whole is easily managed by one man, through 

 control of several electric motors. The counter-weighted dome-floor rises and falls 

 23 feet, on the plan originated by Sir Howard Grubb. The photograph shows it near 

 its lowest position. The dome is 90 feet in diameter, and weighs 140 tons.) 



The -iO-inch glass, too, is in itself so weighty, in consequence 

 of great thickness of both the lenses requisite to accommodate 

 a relatively short tube, that actual flexure of the glass begins to 

 appear as an obstacle to perfect optical performance at varying 

 altitudes. And it is probable that larger objectives of this 

 relatively short focal length would have" to be figured for best 

 performance only within a limited range in zenith distance. 

 Needless to say, too, the protecting dome and the cumbrous 



