4- D. Todd — Open- Air Telescope. 



same is true of refractors, as I found in 1907 when exigency 

 required the mounting of the 18-inch Amherst refractor in the 

 open-air tennis court of Oficina Alianza, in the foothills of the 

 Andes above Iquique, Chile. With a serene atmosphere, 

 cloudless, rainless and windless, it was not surprising that the 

 Expedition brought back to Professor Lowell many thousand 

 photographs of Mars which far surpassed all contemporary pic- 

 tures. The universal excellence of definition at Alianza was 

 largely due to open-air working of the telescope ; and no astron- 

 omer who has once had such an experience would ever insist on 

 operating a telescope from the interior of a dome, except as 

 protection from the weather necessitated it. ' 



Fig. 4. 



Fig. 4. Typical Great Telescope of the 17th Century. (Built and used 

 by Hevelius at Dantzig.) 



The design for a type of open-air telescope which it is the 

 object of this paper to present is the outcome of many years of 

 practical work with both reflectors and refractors. 



The optical advantages of great focal length were known 

 to the earliest telescope makers, especially Huvgens, Cassini 

 (1625-1712), Hevelius (1611-87), and Bia'nchini (1662-1729), 

 whose instruments had single-lens objectives, the harmful 

 aberrations of which were, they found, very greatly reduced 

 by figuring the lens nearly flat, so as to throw the focus remote 

 from the objectglass. But these early makers of long telescopes 



