Page:Scientific Memoirs, Vol. 2 (1841).djvu/63

Rh at Göttingen in the first observations; but has completely ceased since each lamp has been provided with a copper chimney, directed to the side.

As in the term-observations several observers are required, there may be a considerable difference in the distance at which distinct vision is obtained by the several individuals. If a short-sighted person comes to the telescope adjusted for a long-sighted person, some alteration will be required for distinct vision. The use of a concave glass would be inconvenient and unadvisable, on account of the considerable loss of light. The mere sliding of the eye-tube is not sufficient, as, although the image of the scale might thereby be rendered distinct, the cross threads would remain indistinct, and would have a parallax in respect to the image of the object. It would be necessary, therefore, (with the construction which the telescopes employed in these observations usually have) that the cell containing the cross threads should be moveable in the eye-tube, and that it should be brought nearer to the lens in the eye-piece; but this requires a practised hand, takes time, and for other reasons is not to be recommended for the present case. The difficulty may, however, be got over in a very simple manner, if the following plan be adopted. The eye-tube in the telescope, and the cross threads in the same, are to be so adjusted previous to the observations, that the most short-sighted among the observers can see perfectly distinct both the image of the scale and the cross threads; when a longer-sighted person arises in turn, he has merely, without, displacing the eye-tube or the cross threads, to draw out the glass nearest the eye so far that he can define perfectly well the cross threads, and with this a completely distinct vision of the image of the scale is necessarily connected. A short-sighted person coming in turn has merely to make an adjustment in the contrary way.

For the purpose of proving the undisturbed state of the telescope, a mark is employed, which is placed at such a distance that it may be seen distinctly with the same position of the eye-piece as is required for the distinct vision of the image of the scale; this consists, in the Göttingen observatory, of a small vertical line on the northern wall. Previously to the