D. Todd — Open- Air Telescope. 5 



were deprived of realizing the full optical advantage of this 

 construction by the backward condition of : first, the art of 

 making optical glass ; second, the art of figuring and polishing 

 a lens ; third, the undeveloped stage of metal working ; and 

 fourth, the lack of suitable engineering design. (Fig. 4.) 



To any practical astronomer, the almost insivperable difficul- 

 ties in the use of such a telescope would immediately occur. 

 Auzout (1630-91) perhaps built and used a similar one 600 

 feet in length, but I cannot see how he could ever have done 

 either, with the lack of resources of his day. 



The Hall-Dollond (1703-71)-(1?06-61) invention of the 

 achromatic objective did not cancel the desirability of great 

 focal length, and subsequent investigators have not wholly 

 eliminated it. With the longer radii of curvature of the sur- 

 faces, both crown and flint lenses can be reduced to a minimum 

 in thickness with corresponding reduction of the loss of light 

 by absorption. The chief gain of great focal length, as Hast- 

 ings has shown,* is that both spherical and chromatic aberra- 

 tions are more effectively reduced. 



While the parts that make up the complete telescope are but 

 three, the elements that interfere with, and even preclude, its 

 perfect construction and working are essentially but three also. 

 Foremost of all is the ever mobile atmosphere which, though 

 it be transparent, is always quivering and tremulous, never qui- 

 escent; second, the imperfections of work of glassmaker and 

 optician ; and third, insufficient and imperfect design or malcon- 

 struction of mechanical parts of the telescope or mounting, 

 which often interfere seriously with its working, or stand in 

 the way of advance to greater optical power. 



A century ago the 40-foot reflector of Sir William Herschel 

 was in use at Slough, and six-inch object glasses were consid- 

 ered the largest possible. A half-century later Lord Ilosse had 

 finished his 6-foot reflector at Parsonstown, and the Clarks 

 were beginning an 18-J-inch objective, — a truly great advance. 



The years since then have brought unparalleled progress in 

 glass construction, f in optical methods, and even more in the 

 mechanic arts. Telescope builders have advantaged greatly 

 from all this practical progress, culminating in the 40-inch 

 Yerkes telescope (1 897), a noble instrument which has given 

 excellent account of itself in the skillful hands of Barnard and 

 Burnham, Frost, Hale, and Ritchey4 (Fig. 5.) But it is a close 

 approach to the ultimate size attainable in the refracting tele- 

 scope, so long as we confine ourselves to this type of mounting. 



* Hastings, The Sid. Messenger, x, 335, 1891. 

 + Vogel, Astrophys. Jom\, v, 75, 1897. 

 JHale, Astrophys. Jour., vi, 37, 1897. 



