356 ACROPHYSICS. 



the north and south points of the horizon. The direction of a 

 heavenly body, is defined by means of its azimuth, and its altitude, 

 or zenith distance; which latter is the complement of its altitude. 

 This direction is affected by atmospheric refraction, which makes 

 objects appear higher than they really are ; and by parallax, which 

 is the variation that would be produced in the direction, were the 

 object seen from the earth's centre. The refraction is greatest at the 

 horizon, being there nearly half a degree, and it diminishes from 

 thence to the zenith, or point directly overhead, where it is zero, or 

 no refraction takes place. 



The Transit instrument is generally so placed, that the axis of its 

 telescope shall rotate in the plane of the meridian ; which may be 

 found by observing the same star when at equal altitudes, before and 

 after passing the meridian, and bisecting the angle formed by these 

 two positions. We may then find the altitude of the pole, that is, of 

 the point where the earth's axis, prolonged, meets the celestial sphere, 

 by observing the meridian altitudes of a star which is near the pole, both 

 when above and below it, and bisecting the difference, after correct- 

 ing it for refraction. The altitude of the pole is equal to the latitude 

 of the place ; and the longitude may be found by means of a chro- 

 nometer; which is a very accurate timepiece; or by observing an 

 eclipse of one of Jupiter's satellites at that place, and comparing the 

 time with that of the same eclipse at Greenwich ; if the longitude 

 from Greenwich be sought. The difference of time, will give the dif- 

 ference of longitude; allowing 15 for each hour. The sun appears 

 to move eastward among the fixed stars ; but this must be caused by 

 the real motion of the earth around the sun ; for in no other way can 

 all the celestial phenomena be reconciled and explained. 



CHAPTER III. 



OPTICS. 



OPTICS, is that branch of Acrophysics, or Natural Philosophy, 

 which treats of the nature and properties of light, and the phenomena 

 of vision. The name is derived from the Greek, oTt^ou, I see. 

 Light, is an emanation, or something proceeding from bodies ; 

 which, reaching the eye, makes a peculiar impression, as sound does 

 upon the ear; an impression to which the other organs of the human 

 body are insensible. Light is found to move generally in straight 

 lines; and with a velocity of 192,500 miles per second; or from 

 the sun to the earth in about 8 minutes. A ray of light, is compre- 

 hended in a single line proceeding from any luminous point ; and an 

 assemblage of rays forms a beam, or pencil, of light. When light 

 impinges on any body, it is either reflected from it, or transmitted 

 through it, or absorbed within it ; and these effects are frequently 

 produced conjointly. These facts give rise to the old divisions of 

 Catoptrics, treating of reflected light, and Dioptrics, treating of trans- 

 mitted light ; which, however, do not exhaust the subject. 



