WITH A FOURTEEN FEET REFLECTOR. 181 



The various particulars in the figured nebulse were confirmed by Mr. Smith on different 

 evenings; and such points as he did not confirm are considered as uncertain.* 



MiCROMETRICAL MEASURES OF THE StARS IN THE NeBUL2E. 



20. The instrument with which these were taken was the ten feet telescope 

 of G. Dollond, belonging to the philosophical department of Yale College. 

 The aperture of this telescope is five inches; and its defining power is such as 

 to resolve £ Librae, a Coronse Borealis, and other double stars of between 1" and 

 2" distance, and to exhibit the division in the ring of Saturn with ease. The 

 micrometer is also of Dollond's most recent construction; a parallel wire mi- 

 crometer, with screws of about one hundred threads to the inch, each revolu- 

 tion of the screw being subdivided into lOOths upon graduated heads. Farther 

 subdivision, when desirable, is easily made by estimation. The wires are, I 



* The fourteen feet telescope used in these observations was taken down in the latter part of August. On the 

 graduation of the class in Yale College to which Mr. Smith and myself had belonged, the reflector became wholly 

 the property of Mr. Smith, and was removed to Ohio City, Ohio. It has already been remounted there, and in a 

 better frame, and with more perfect adjustments than before, as an extract from a recent letter of his will show: 



" The stand," he writes, " is precisely similar to the engraving, in the Philosophical Transactions, of the one 

 erected by Ramage at the Royal Observatory, Greenwich. It is full twenty feet high, and has a sliding gallery 

 that will hold, with convenience, four persons. The telescope swings between two parallel beams, about six feet 

 apart; up and down these the gallery can be raised or depressed by one person easily. The telescope is con- 

 structed to roll forward, so that the mouth may be at a convenient distance from the gallery. After observing, 

 the tube can be let down into a tight bpx, and locked up; the whole is supported by four iron rollers, and can be 

 turned by one person in any direction. The apparatus for slow motion in altitude is the same as in New Haven; 

 I have been making some arrangements for a slow motion in azimuth." Mr. Smith farther writes that he has 

 been prevented from making any observations of importance by the continuance of unfavourable weather. He 

 has, however, lately fitted an excellent micrometer to his telescope, the work of an American artist, and will soon 

 be prepared for fresh labours. From his zeal and activity, with so valuable an instrument, we cannot but expect 

 the most interesting results. By the same letter I am furnished with a note of his observations on the 16th of 

 July;, 1839, which I insert as favourably exhibiting the power and capability of the telescope about the time it 



was employed on the foregoing nebulae " July 19, 8, P.M. Saw y Virginis with 400 and 700, the 



latter a single lens, very beautifully; the same evening, 10, P. M , saw k Ophinchi distinctly separated, and at 

 11, P. M., ^ Bootis triple." The latter I have never seen with this telescope, nor ^9 Hootis, which Mr Smith 

 says he has " frequently seen," but adds " that he could never separate » Coronse Borealis." These afford good 

 tests of the defining power of the instrument. 



But few of the common tests of light have been observed. Favourable specimens of its "minimum visibiU" 

 are, however, (149) and (15G) of the large Nebula Cygni, already instanced, besides two or three of the smallest 

 ^tars in each of the other nebulae. By an eniry in the rough journal of observations, it appears, that k. 250, or 

 the " Polarissima" of Herschel, a nebula almost exactly at the pole, was, if certainly seen, a fair example of it« 

 Hmit of visual power. 



VII. — 2 V 



