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III.—To adjust the Polar Axis to the Latitude of Observation.

Read the declination of a star as nearly as possible on the meridian; if this is the same as the declination given in the Nautical Almanac or a star catalogue, the position of the polar axis is correct. If the two amounts do not agree, set the vernier to the correct catalogue declination, and bring the star into the centre of the field of view by altering the latitude screw in the iron foot, moving the telescope in right ascension as requisite.

Example:—The declination of a Leonis was read as 130 4' N., the catalogue showed that it ought to have read 120 36', the telescope was lowered so that the vernier read 120 36', and the latitude screw was altered until the star was central.

IV.—To test the accuracy in position of the Telescope.

A. —Should the declination of a star six hours from the meridian, that is, due east or west, read correctly in both positions, as well as at transit, the position of the telescope is correct.

B. —Should meridian declinations read correctly, while at equal distances from that position there are equal errors of a contrary nature (that is, one + and the opposite —), the vernier is correctly set, but the meridian position of the instrument is wrong.

C. —Should meridian declinations read correctly, while at equal distances from that position there are equal errors of the same nature (that is, both + or both —), the vernier is not correctly set, but the meridian position is right.

D. —Should meridian declinations read correctly, while at equal distances from that position, there are unequal errors of any nature, both the setting of the vernier and the meridian position are incorrect.

A plus error (+) means that the vernier reading is higher than the catalogue position ; for example, the reading is io° N. and the catalogue gives 8° 30' N., or if of south declination 50 S. to catalogue io° S. A minus error (—) of course means exactly the contrary. But for all adjustments, stars of north declination should be selected : for meridian positions a moderate altitude will suffice, but for observations at six hours from the meridian only stars that are above 250 N. declination should be used. The reason why stars are observed at or about six hours' distance is that an horizontal movement given to the foot of the equatorial will cause the telescope, if pointed in that direction, to move in declination; such stars may, for brevity, be called six hour stars.

The method by which the vernier of a German equatorial can be accurately set having been already described and presumably acted on, its position will not require correction. But its correct position

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