D. Todd — Open-Air Telescope. 



brought to light unexpected features of these primal cloud- 

 like forms, without minute optical knowledge of which we can 

 scarcely hope ever to know their full siguificauce in the 

 cosmogony. These are but two of the "new lights" that await 

 the present-day astronomer's interpretation. 



All telescopes are made up of three parts : Objectglass, eye- 

 piece, and some form of rigid mechanical connection between 

 the two. Usually this last is a round tube, either cylindrical 

 or conical ; but there is no optical reason why it may not take 

 any shape that the exigencies of mechanics or engineering may 

 demand. 



Fig. 2. , 



Fig. 2. Sir William Hersckel's 40-ft, Telescope. 

 Patron, King George the Third, 1795.)* 



(Inscribed to his Eoyal 



Many open-tube telescopes have been built. Huygens 



(1629-95) even built one with no tube at all, an aerial telescope, 



as he called it; the objective in a counterbalanced cell with 



universal joint, mounted on top of a tall pole. He drew the 



*Herschel, W.. Phil. Trans, for 1795, p. 347. 



