io6 



NATURE 



[April io, 19 19 



under constant temperature. A similar test after a 

 'daytime rise of about 1*5° C. showed longitudinal 

 aberration of only about 0*5 mm. This corre- 

 sponds with a very good figure, and it was evident, 

 if the best results were to be obtained, that some 

 means should be adopted for reducing the change 

 of temperature around the mirror. This was 

 effected by permanently covering the closed sec- 

 tion of the tube, consisting of two steel castings 

 weighing more than 10 tons, with felted cotton 

 about 2 in, thick. The space between the back 

 and edges of the mirror and the bottom and sides 

 of the cell was also packed with 

 this felt; and a removable pad 

 placed on thin boards laid across 

 the top of the closed section, 

 when the mirror is not open to 

 the sky, completes the enclosure 

 of the mirror. 



During the daytime rise of 

 temperature in the dome the 2 

 tons of glass in the mirror and 

 the 10 tons of steel in the centre 

 section and cell of the tube are 

 protected by this heat-insulating 

 material, with the result that the 

 change of temperature around 

 the mirror is very slow. This 

 change amounts to only about 

 one-third of that in the dome, 

 and does not often exceed 1° C, 

 with the result that the aberra- 

 tion is always so small as to be 

 negligible in increasing the size 

 of the star image in comparison 

 with the enlargement caused by 

 atmospheric disturbances. The 

 quality of the optical parts and 

 their performance since the insu- 

 lating cover was applied leave 

 nothing to be desired, and 

 although only a few direct photo- 

 graphs . at the principal focus 

 have yet been made, the defini- 

 tion is superb, the smallest 

 images being but slightly more 

 than a second of arc in diameter. 

 For use with the spectrograph a 

 Cassegrain combination is em- 

 ployed : the principal mirror of 

 72-in. aperture and 30-ft. focus, 

 and the convex secondary of 

 20-in. aperture placed about 

 7 ft. within the principal focus. The resulting 

 equivalent focal length is 108 ft., and it is 

 sufficient evidence. of both the optical quality and 

 the seeing conditions to say that frequently the 

 greater part of the star image appears to be lost 

 in the spectrograph slit, which is 0*3 mm. wide, and 

 that a spectrum, of linear dispersion 35 A. to the 

 mm. at Hy, of a star of 7*0 magnitude can be 

 obtained in 20 to 25 minutes under average seeing 

 conditions. 



However good the optical parts, effective work 

 could not be done unless the mechanical parts 

 NO. 2580, VOL. 103] 



were so designed and constructed as rigidly and yet, 

 flexibly to carry the optical parts in their correct 

 relative positions, and the mechanism so arranged 

 as to enable the telescope to be pointed quickly and 

 accurately to the desired position, and then 

 to follow accurately the apparent motion of the 

 star. This has been effected in the telescope in 

 a remarkably efficient manner, and I have no hesi- 

 tation in saying that this mounting sets a new 

 standard for convenience and accuracy in opera- 

 tion. The telescope is set and guided by electric 

 power having three speeds in each co-ordinate : 



-The telescope from the north-west. 



a quick motion of 45° per minute, a fine setting 

 motion of 10' per minute, and a guiding speed 

 of o"5' per minute, one revolution in 8 minutes, 

 36 hours, and 30 days respectively. The quick 

 motion and clamps are operated from duplicate 

 switchboards on each side of the south pier (one of 

 these can be seen in Figs. 2 and 3), while the fine 

 setting and guiding is done from a small, portable 

 board carried by the observer. No fine<:ircles are 

 provided, but the coarse circles are graduated to 

 minutes of time in R.A., and to 5' in declination. 

 In consequence, the telescope can be set easily and 



