59 



solar telescope ; then revolve the solar telescope on its polar axis 180 to see if its 

 bubble remains in the center of its tube : if not, remove half its error by means of the 

 opposing tangent screws, the other half by the milled capstan-headed screws below 

 the base-plate, until it remains in the center of the tube. Repeat if necessary. 



Turn the solar telescope 90 on its polar axis, and by the milled capstan-headed 

 screw level the base-plate and bring the bubble to the center of the tube. Repeat 

 the operation until the bubble of the solar telescope remains in the center of the tube 

 upon revolving the solar telescope around its polar axis. (This condition must be 

 attained before the polar axis can be set to the co-latitude of the observer ; and being 

 attained it needs no further attention than being examined at times for verification). 



The adjustment of the polar axis to be truly at right angles to the line of sight of 

 the main telescope is made by two milled capstan-headed screws and two opposing 

 springs at 'right angles to each other below the base or leveling plate of the solar 

 attachment. As will be seen in making this adjustment it is not necessary to place 

 the solar telescope parallel or at right angles to the main telescope, but simply in the 

 same vertical plane of each set of leveling screws and springs at the time. This 

 adjustment is made by the manufacturer and thereafter needs only to be examined 

 at times. 



If the adjustments are properly made the bubble of the level of the transit telescope 

 and those of the plate levels on the transit will all be in the center of their tubes, and the 

 vertical arc will read zero. 



Bisect some convenient object, and turn the solar telescope sufficiently to the right 

 or left, around the polar axis, to make the image of the object traverse the field from 

 one side of the tube to the other. The image should remain bisected by the wire, 

 If not, loosen the four capstan-headed screws of the diaphragm till the above condition 

 is attained, and fasten the screws securely, 



The solar telescope showing usually objects inverted, requires the cross-wire dia- 

 phragm to be moved as described in the foot-note on page 49 of Manual. 



Bisect any very distant object in the horizontal plane by the main telescope, and 

 clamp. Then, by means of the clamp and opposing tangent screws on the solar teles- 

 cope, bring its horizontal cross-wire to bisect the same object ; then, by means of the 

 capstan-headed screw of the solar telescope level bring the bubble to the middle of 

 its tube. This being done, the optical axes of the two telescopes will lie in parallel 

 planes for distant objects* and the instrument is ready for use. 



All these adjustments are made by the manufacturer, and need to be verified only 

 occasionally. 



Before the solar attachment is available for finding meridian, the observer must 

 know his Latitude, and the sun's Declination for the day and hour of observation, 

 corrected for refraction, whence the 



Reduction of Declination and Refraction. 



The sun's Declination is given for noon of every day in the year, in the Washing- 

 ton and Greenwich Ephemeris of the sun, for those meridians. The maps and charts 

 in use will give.the difference of Longitude to all the precision required, and tables in 

 this manual give the required Refraction. 



An example will best illustrate : 



Required a declination table for the different hours of the day for April 25, 1885. 

 Lat. 44 N., and Longitude 97 P W. At 15 to the hour, 97 of longitude is about 

 6 1 hours of time, and as this longitude is W., 12 o'clock, or noon, at Greenwich will 

 correspond to 5| A. M. at the place of the observer. 



The declination, as given for that day, in the Greenwich Ephemeris, is 13 20' 

 04" N., and is shown to be gaining at the rate of 49" per hour (see column headed 

 Difference for one Hour, with the signs -4- for sun going North, and for sun 

 going South). 



* For use of the solar telescope as a top telescope in mine work when nearer objects at variable distances 

 are viewed, it will be necessary to first make the adjustment of the cross-wires for collimation by rotating the 

 solar telescope in improvised wyes, as explained under " Patent Adjustable Top Telescope," page 106, in 

 order to make the line of collimation correct for all distances. 



