D. Todd— Open-Air Telescope. 3 



)f the objective into approximate line with the axis of 

 the ocular by means of a small cord or wire reaching clown to 

 the observer on the ground. No objective of modern optical 

 excellence could ever be satisfactorily used in this way. The 

 4-foot Melbourne reflector designed and built by Thomas Grubb 

 (1800-78) has an open tube of spirally interlacing straps of 

 steel. (Fig. 1.) 



Fig. 3. 



Fig. 3. Lord Eosse's Great six-foot Eefiecting Telescope. 

 Parsonstown, Ireland.)* 



(Birr Castle, 



The superior performance of a great telescope in the open air 

 has Ions been known to astronomers. The huge instruments 

 of Sir William Herschel (1738-1822) (fig. 2), of Lord Eosse 

 (1800-67) (fig. 3), and of Lassell (1799-1880),f were all open-air 

 instruments. All these were reflectors of course ; but the 



* Eosse, Phil. Trans., cli, 6S1, 1861. 



\ Lassell, Mem. Eoy. Astron. Soc, xxxvi, 1867. 



