304 SCIENTIFIC RECORD FOR 1882. 



'' The solar observatory was destined by Professor Watson for two pur- 

 poses: First, spectroscopic and photographic observations of the sun 

 itself; and, second, for observations of the immediate neighborhood of 

 the sun for the detection of a planet (Vulcan) interior to Mercury. 



"ThemaguitudeofYulcan was estimated by Professor Watson atfour 

 and one half, that is, it has a light about one-twentieth as brilliant as that 

 of an average first-magnitude star — Alpha Lyr?e, for example. If seen 

 at all, it must be seen in the daytime, and close to the sun. 



" During the months of June and July experiments were made to de- 

 termine the fitness of the observatory for the detection of Vulcan. In 

 these experiments the six-inch telescojie bought by your Board from Mr. 

 Burnham was mounted in the cellar of the solar observatory on June 8 — 

 its objective and eyepiece having been carefully adjusted previously. To 

 get an idea of the advantage gained by the inclined tube of the solar 

 observatory I reduced the aperture of the fifteen-inch telescope in the 

 dome to six inches, and put on an eyepiece similar to that used on the 

 small refi'actor. Both telescopes were then pointed to the north pole. 

 Two observers, one at each telescope, noted times of the first a])pear- 

 ance of the small stars about the north pole as evening twilight dis- 

 appeared. A map had been prepared previously giving these stars. If 

 the fifty five-foot dewcap gave a material advantage, then these stars 

 should have been first seen with the solar observatory telescope. In 

 fact, they were first seen by this, but only by about two minutes on the 

 average. This experiment was tried on June 8, 11, and 12. 



" The excellent heliostat lent by Dr. S. P. Langiey, director of the 

 Allegheny Observatory, was placed on its ])ier June 12, and adjusted. 

 This instrument was exactly fitted for the purpose, and was in perfect 

 order. 



" Observations were made to find stars in the daytime every clear day 

 between June 15 and Jnly 12, without result. The place in the sky to 

 which the heliostat mirror was pointKl was fixed by setting on the sun 

 at a given hour ; and the fact that this setting was not changed by ac- 

 cident was established by observing the position of a small pointer 

 which was added to the heliostat. 



"The latter trials were made with a heliostat mirror, set for the place 

 of the brightest star in the Pleiades, and on three days this group was 

 looked for about the time of its passing the meridian. Forty or fifty 

 minutes were thus spent daily. 



" There must have been in the field of the telescope (30 degrees) the fol- 

 lowing stars : 



" Eta Tauri — 3.4 mag. (twice as bright as Vulcan). 



"6 Pleiadum — 4.5 mag. (as bright as Vulcan). 



" d Pleiadum — 5 mag. (fainter than Vulcan). 



"/ Pleiadum — 4.5 mag. (as bright as Vulcan). 



" h Pleiadum — 5.6 mag. (fainter than Vulcan). 



"At this time the sun was some 50 degrees east of the Pleiades. Xo 

 stars were seen at any lime. The position of the mirror for the Pleiades 

 was verified by finding a group of stars at night-time in the j)rediGted 

 place. The original setting was made' through the sun. 



"I am satisfied, therefore, that there is no use in prosecuting this par- 

 ticular experiment further. The instruments employed were as perfect 

 as possible. Every conceivable precaution was taken, and it was shown 

 that this apparatus was not suitable for seeing stars of the magnitude 

 of Vulcan, even distant from the sun. It would, therefore, be a waste 

 of time to look for such stars close to the sun. 



"It is to be ncticed that no evidence has been collected in regard to 



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