[Illustration: FIG. 93.—Diagram of Snow Horizontal Telescope.] A telescope pointed at such a mirror will hold the stars motionless in its field as if the firmament were halted à la Joshua. But if a change of view is wanted the telescope must be shifted in altitude or azimuth or both. This is altogether inconvenient, so that as a matter of practice a second plane mirror is used to turn the steady beam from the cœlostat into any desired direction. By thus shifting the mirror instead of the telescope, the latter can be permanently fixed in the most convenient location, at the cost of some added expense and loss of light. Further, the image does not rotate as in case of the polar heliostat, which is often an advantage. An admirable type of the fixed telescope thus constituted is the Snow telescope at Mt. Wilson (Cont. from the Solar Obs. #2, Hale). Fig. 93 from this paper shows the equipment in plan and elevation. The topography of the mountain top made it desirable to lay out the axis of the building 15° E. of N. and sloping downward 5° toward the N. At the right hand end of the figure is shown the cœlostat pier, 29 ft. high at its S end. This pier carries the cœlostat mirror proper, 30 inches in diameter, on rails _a a_ accurately E. and W. to allow for sliding the instrument so that its field may clear the secondary mirror of 24 inches diameter which is on an alt-azimuth fork mounting and also slides on rails _b b_. The telescope here is a pair of parabolic mirrors each of 24 inches aperture and of 60 ft. and 145 ft. focus respectively. The beam from the secondary cœlostat mirror passes first through the spectrographic laboratory shown to the left of the main pier, and in through a long and narrow shelter house to one of these mirrors; the one of longest focus on longitudinal focussing rails _e e_, the other on similar rails _c c_, with provision for sliding sidewise at _d_ to clear the way for the longer beam. The ocular end of this remarkable telescope is the spectrographic laboratory where the beam can be turned into the permanently mounted instruments, for the details of which the original paper should be consulted. The purpose of this brief description is merely to show the beautiful facility with which a cœlostatic telescope may be adapted to astrophysical work. Obviously an objective could be put in the cœlostat beam for any purpose for which it might be desirable. Such in fact is the arrangement of the tower telescopes at the Mt. Wilson Observatory. In these instruments we have the ordinary cœlostat arrangement turned on end for the sake of getting the chief optical parts well above the ground where, removed from the heated surface, the definition is generally improved. To be sure the focus is at or near the ground level, but the upward air currents cause much less disturbance than the crosswise ones in the Snow telescope.