The elements of astronomy; a textbook . Fig. Pyrheliometer. §557] THE EQUATORIAL COUDE. 426 a 557. The Equatorial Coude. —A great telescope mounted inthe usual manner is very unwieldy, and various arrangementshave been devised for relieving the difficulties. In the Equa-torial Coude, or Elbowed Equatorial, now in use at Parisand in several other observatories, the inconveniences areovercome by the use of two mirrors, one outside of the object-glass, the other in the angle where the tube joins the polaraxis, as shown in the figure. The observer sits in a fixed posi-. Fig. 159. —Th
The elements of astronomy; a textbook . Fig. Pyrheliometer. §557] THE EQUATORIAL COUDE. 426 a 557. The Equatorial Coude. —A great telescope mounted inthe usual manner is very unwieldy, and various arrangementshave been devised for relieving the difficulties. In the Equa-torial Coude, or Elbowed Equatorial, now in use at Parisand in several other observatories, the inconveniences areovercome by the use of two mirrors, one outside of the object-glass, the other in the angle where the tube joins the polaraxis, as shown in the figure. The observer sits in a fixed posi-. Fig. 159. —The Equatorial Coude. tion, under shelter, looking down through the polar axis, whichis hollow and forms part of the telescope tube. The instru-ment is of course much more expensive than the usual form;but the costly revolving dome is dispensed with. There is a considerable loss of light by the two reflections, andanother, much simpler form with only one mirror is now in use atCambridge (England). It commands, however, only a limitedportion of the heavens, from — 30° to 4- 75° of declination. 426 b APPENDIX. [§558 558. The Siderostat. — In various lines of astronomicalwork, photographic and spectroscopic especially, there aregreat advantages in having the telescope and its accessoriesmounted solidly upon independent piers in a convenient posi-tion, the object under observation being reflected into it steadilyby a plane mirror suitably moved by clock-work. The Sidero-stat of Foucault consists of a mirror with a cylindrical stemprojecting from its back, and so mounte
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