48

Table of Refractions (Bessel).

Altitude. Refraction. Altitude. Refraction. Altitude. Refraction. Altitude Refraction.

o° 34' 24' 54" 6° 8' 2 3" Hq 3' 47" 35° T 22"

1° 24" 7° 7' 19" 16° 3' 2' 18" 4°° T 8"

2° 18' 8" 8° 6' 29" x8° 56" 5o° 48"

3° H' 14" 9° 5' 49" N O O 2' 3 7" 6o° 33"

4° iT 39" IO° 5' 16" 25° 2' 3" 7°° 21"

5° 9' 46" 12° 4' 25" 0 0 cn T 39" 8o° 10"

Thus a star which really was on the horizon would appear 34' 54" above it.

When an object is some distance from the meridian, refraction influences right ascension as well as declination. The observer need not make any allowance for refraction except when finding objects with high powers on an achromatic telescope, or in ascertaining their exact position. With a low power the object sought is sure to be in the field without allowing for any refraction.

To adjust the “Finder.”

Direct the Telescope to any bright star (the Pole-star being by far the best, as it has very little apparent motion), and bring this star into the centre of the field of a low-power eye-piece. Now adjust the “ finder ” by means of the three screws bearing on it, till the star is bisected by the cross wires seen in the focus of the eye-lens of the “ finder.” Change the low eye-piece to a high one, and perfect the adjustment in the same way. Any well defined terrestrial object at a distance can be employed, in the day time, to roughly adjust the “ finder,” leaving the final adjustments to be made on a star. If a small Newtonian is used as a “finder,” its line of sight may be regulated by slightly altering the adjustment of the speculum of the “ finder,” by means of the three screws.

With reflectors the finder is often made of sufficient power to act as an equatorial telescope. After the optical adjustments of a reflector have been perfected the finder is accurately adjusted, and then the setting of the equatorial is done with the finder alone. The great advantage of this plan is that the finder can be firmly fixed, and the mirrors removed, for silvering or otherwise, and afterwards replaced and adjusted to the finder, thereby running no risk of disturbing either the optical or equatorial adjustments of the instrument.

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Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 97,12 %.

La langue de reconnaissance de l'OCR est l'Anglais.

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