Page:Popular Astronomy - Airy - 1881.djvu/57

Rh approach nearly to a flat with the horizon; and if S′O or SG approach nearly to a perpendicular to the horizon, OG′ will approach nearly to a perpendicular to the horizon. These are the facts upon which the use of observations by reflection is founded. If we place this small trough in such a position that the telescope looks into it, and if we see a star, we know that the light which comes from that star is reflected in such a manner, that the position of that telescope must be as much inclined downwards, as the position of the direct ray of light from the star is inclined upwards. From these observations we infer the position of the telescope when it is horizontal I then pointed out to you that by the use of this, we ascertain the elevation of the Polar Star at its highest and its lowest positions, and that by taking the mean of these we have the height of the Pole; and therefore, getting the elevation of the Pole on one side of the Zenith, and getting the elevation of any other star or any object whatever, passing either on the same side of the Zenith or on the other side; by means of these we ascertain the angular distance of any object from the Pole.

Incidentally, I had occasion to point out to you some other things. One of these was the use of the wires in the telescope. And I constructed as it were a telescope, Figure 7, though it has no tube. The telescope I have here made consists of a lens, representing the object glass of a telescope, and a screen, representing the field of view. Formerly telescopes were made without any tubes at all, so that the tube of a telescope is totally unessential. The construction I have got there is a proper representation of a telescope, forming an image of the object on the screen, which screen is in the place in which you