628 



SCIENCE. 



[N. S. Vol. XII. No. 304. 



inclies in diameter. The lower half of the 

 observing slit was fitted with a screen of tar- 

 paulin by Professor Keeler, which proved 

 very useful against the winds which are so 

 prevalent on Mt. Hamilton. 



The telescope and its mounting are very 

 completely described and figured by Dr. 

 Common in Vol. 46 of the Memoirs of the 

 Royal Astronomical Society. The telescope 

 was designed especially for photographic 

 work, in which it is very desirable to con- 

 tinue an exposure uninterrupted, across the 

 meridian. This cannot be done in the usual 

 form of mounting where the declination axis 

 is attached to the middle of the tube of the 

 telescope and to one end of a short polar 

 axis. Dr. Common obviated this difiiculty 

 by placing the declination axis at the ex- 

 treme lower end of the telescope tube, the 

 axis of the telescope always being in the 

 same meridian plane as the polar axis. The 

 large mirror is above the declination axis, 

 and hence requires to be counterbalanced. 

 This counterbalancing of mirror and tube 

 is effected by placing slabs of lead in two 

 boxes which extend a short distance be- 

 yond the declination axis. Professor Keeler 

 points out that the construction adopted by 

 Dr. Common for his 5-foot reflector is a 

 great improvement. In the latter instru- 

 ment the tube is swung near its lower end 

 between two large ears attached to the 

 polar axis, the pivots forming the declina- 

 tion axis. The mirror is placed at the ex- 

 treme end of the tube and thus acts as a 

 counterbalance. 



The construction of the mounting limits 

 observations to 25° south declination. In 

 London, for the latitude of which the mount- 

 ing was constructed, this meant a zenith 

 distance of 77° on the meridian, but atMt. 

 Hamilton it is only 62 ° . In our more south- 

 ern latitude a considerable region is thus 

 unfortunately out of reach of the telescope. 

 From his experience with the Crossley 

 reflector, Professor Keeler came to the con- 



clusion that the definition in the case of a 

 reflector, as well as a refractor, depends al- 

 most wholly on external conditions, and that 

 large masses of metal near the mirror have 

 little effect, at least where the range of 

 temperature is as small as at Mt. Ham- 

 ilton. 



The telescope is used as a Newtonian. It 

 is provided with two large mirrors, each 

 three feet in aperture, and of 17 feet 6.1 

 inches focal length. These mirrors were 

 made by Mr. Calver. The one in use at 

 present (mirror A) was refigured by Sir 

 Howard Grubb, and is practically perfect 

 for photographic work. It may be added 

 that this mirror has so far been used with 

 the same coating of silver which it had 

 when received from England. Mirror B 

 has not been used at Mt. Hamilton. 



The diagonal mirror is round, its diameter 

 being 8.9 inches. Its distance inside the 

 focus is 29 inches. 



The field of view after reflection is, there- 

 fore, elliptical. Its mounting is such, how- 

 ever, as to cut off an almost circular sec- 

 tion of rays from the large mirror. 



The tube of the telescope is a square 

 framework of iron tubes braced by diagonal 

 rods and is provided with curtains of black 

 cloth to close the tube in. Professor Keeler 

 found that any fogging of the plate was due 

 to diffused skylight and the curtains have 

 therefore been dispensed with. The outer 

 end can be rotated about the axis of the 

 telescope to bring the eyepiece into as com- 

 fortable a position as possible. This section 

 carries the diagonal mirror and the eye end. 

 The latter has the customary arrangement 

 for focusing and is made to take the short 

 tubes, one of which is arranged for visual 

 and the other for photographic work. The 

 photographic slide contains, in addition to 

 the double motion device, an adjustable 

 slide carrying an eye-piece with cross wires 

 for guiding. This is placed very close to 

 the plate-holder slide and clamped to it. 



