Swasey Observatory 351 



a dial which is made to revolve in sidereal time by the equa- 

 torial driving clock. A double pointer on this dial is made to 

 move in unison with the polar axis, thus enabling the observer 

 to directly point the telescope to the right ascension of the star 

 to be observed, with the same ease that it is set on the declination 

 of the star. Under this sidereal dial on the north side of the 

 column is a wheel with handles, connected with gearing to the 

 polar axis, enabling the observer to easily set the telescope while 

 reading the sidereal dial. 



The optical parts of the telescope were all made by the John 

 A. Brashear Company, of Pittsburgh, Pa. The objective has a 

 clear aperture of 9" and a focal length of 135'^; and the finder 

 has a clear aperture of 3" and a focal length of 17V'. 



In addition to the usual number of eye-pieces there is pro- 

 vided a diagonal prism for zenith observations ; a helioscope for ob- 

 serving the sun, and a fine micrometer for measuring double stars. 



The 4" combined transit and zenith instrument is of the stand- 

 ard design with iron base and columns. It is provided with a 

 universal micrometer with electric illumination showing dark 

 wires on a bright field. The axis carries two circles 12" in dia- 

 meter, one to be used as a setting or finding circle, reading by oppo- 

 site verniers to single minutes ; the other divided upon coin silver 

 with a fine level to read to ten seconds by two double opposite 

 verniers. 



The instrument is provided with a most perfect system of 

 reversal, enabling it to be used not only as a transit instrument but 

 as a zenith telescope. 



The cylinder chronograph is provided with a driving clock 

 controlled by a conical pendulum similar to that used in the 9" 

 telescope. It has a drum 7" in diameter, and 14" in length, on 

 which is recorded the work done by the transit instrument. A 

 carriage carrying the pen which makes the records is connected 

 electrically with the sidereal clock, thereby making a record on 

 the drum every second. From the same carriage electric wires 

 are carried to the transit instrument, the contact being made by 

 the observer as the star which is being observed crosses the spider 

 lines in the micrometer. By this means a record within one-tenth 

 of a second is permanently made on the sheet of paper carried 

 by the drum. These records of the observer made on the drum 

 during an evening can be computed at leisure by the astronomer. 



