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SCIENCE. 



[N. S. Vol XII. No. 303. 



importance are confined to the mounting of 

 the necessary machinery to carry the large 

 plane mirror and move it round at the 

 proper rate. The telescope need not have 

 any tube (that to the Paris telescope is of 

 course only placed there for effect), as the 

 flimsiest covering is enough if it excludes 

 false light falling on the eye end; and more 

 important than all, the observer sits at his 

 ease in the dark chamber. This question 

 of the observer, and the conditions under 

 which he observes, is a most important one 

 as regards both the quality and quantity of 

 the vsrork done. 



We have watched the astronomer, first 

 observing from the floor level; then mounted 

 on a high scaffold like Sir William Herschel, 

 Lassell, and Lord Kosse; then, starting 

 again from the floor level and using the 

 early achromatic telescope ; then, as these 

 grew in size, climbing up on observing chairs 

 to suit the various positions of the eye end 

 of the telescope, as we see in Mr. Newall's 

 great telescope ; then brought to the floor 

 again by that excellent device of Sir Howard 

 Grubb, the rising floor. This is in use with 

 the Lick and the Yerkes telescopes, where 

 the observer is practically always on the 

 floor level, though constant attention is 

 needed, and the circular motion has to be 

 provided for by constant movement, to say 

 nothing of the danger of the floor going 

 wrong. Then we have the ideal condition, 

 as in the Equatorial Coud6 at the Paris 

 Observatory, where the observer sits com- 

 fortably sheltered and looks down the tele- 

 scope, and from this position can survey the 

 whole of the visible heavens. The comfort 

 of the observer is a most important matter, 

 especially for the long exposures that are 

 given to photographic plates, as well as for 

 continued visual work. In such a form of 

 telescope as that at Paris the heliostat form 

 of mounting the plane mirror is most suit- 

 able, notwithstanding the rotation of the 

 image. But there is another way in which 



a plane mirror can be mounted, and that is 

 on the plan first proposed by Auguste many 

 years ago, and lately brought forward again 

 by Mons. Lippman, of Paris, and that is 

 by simply mounting the plane mirror on a 

 polar axis and parallel therewith, and caus- 

 ing the mirror to rotate at half the speed 

 of the earth's rotation. Any part of the 

 heavens seen by any person reflected from 

 this mirror will appear to be fixed in space, 

 and not partake of the apparent movement 

 of the earth, so long as the mirror is kept 

 moving at this rate. A telescope, therefore, 

 directed to such a mirror can observe any 

 heavenly body as if it were in an absolutely 

 fixed position , so long as the angle of the mir- 

 ror shall not be such as to make the reflected 

 beam less than will fill the object-glass. 

 There is one disadvantage in the ccelostat, 

 as this instrument is called, and that is its 

 suitability only for regions near the equator. 

 The range above and below, however, is 

 large enough to include the greater portion 

 of the heavens, and that portion in which 

 the solar system is included. Here the 

 telescope must be moved in azimuth for 

 different portions of the sky, as is fully ex- 

 plained by Professor Turner in Vol. LVI. 

 of the Monthly Notices and it therefore 

 becomes necessary to provide for moving 

 the telescope in azimuth from time to time 

 as different zones above or below the equator 

 are observed. No instrument yet devised 

 is suitable for all kinds of work, but this 

 form, notwithstanding its defects, has so 

 many and such important advantages that 

 I think it will obviate the necessity of build- 

 ing any larger refractors on the usual mod- 

 els. The cost of producing a telescope much 

 larger than the Yerkes on that model, in 

 comparison with what could be done on the 

 plan I now advocate, renders it most im- 

 probable that further money will be spent 

 in that way. It may be asked : What are 

 the lines of research which could be taken 

 up by a telescope of this construction, and 



