670 REPORT— 1900. 



tlie Paris telescope is of course only placed there for effect), as tlie flimsiest 

 coveriDg 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 work done. 



"\Ve have watched the astronomer, first observing from the floor level, then 

 mounted on a high scaffold like Sir William Herschel, Lassell, and Lord llosse ; 

 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 ibr by constant movement, to 

 say nothing of the danger of the floor going wrong. Then we have the ideal 

 condition, as in the Equ.atorial Coude at the Paris Observatory, where the observer 

 sits comfortably sheltered and looks down the telescope, 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 photo- 

 graphic plates, as well as for continued visual work. In such a form of telescope 

 as that at Paris the lieliostat form of mounting the plane mirror is most suitable, 

 notwithstanding the rotation of the image. But there is another way in which 

 a plane mirror can be mounted, namely, on the plan first proposed by Auguste 

 many years ago, and lately brought forward again by Mods. Lippmann, of Paris, and 

 that is by simply mounting the plane mirror on a polar axis and parallel therewith, 

 and causing this 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 flxed 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 mirror shall not be such as to make the reflected beam 

 less than will fill the oliject-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 diH'erent portions of 

 the sky, as is fully explained by Professor Turner in vol. Ivi. of the ' Monthly Notices,' 

 and it therefore becomes necessary to provide for moving the telescope in azimuth 

 from time to time as ditierent zones above or below the equator are observed. 

 JSTo instrument yet devised is suitable for all kinds of work, but this form, not- 

 withstanding its defects, has so many and such important advantages that I 

 think it will obviate the necessity of building any larger refractors on the usual 

 models. The cost of producing a telescope much larger than the Yerkes on that 

 model, in comparison with what could he done on the plan I now advocate, renders 

 it most improbable 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 on what lines should the telescope be built ? I will endeavour 

 to answer this. All the work that is usually done by an astronomical telescope, 

 excepting very long-continued observations, can be equally well done by the fixed 

 telescope. But there are some special lines for which this form of research is 

 admirably suited, such as photographs of the moon, which would be possible with 

 a reflecting mirror of, say, 200 feet focal length, giving an image of some 2 feet 

 diameter in a primary I'ocus, or a larger image might be obtained either by a longer- 

 focus mirror or by a combination. It might even be worth while to build 

 a special coelostat for lunar photography, provided with an adjustment to the 

 polar axis and a method of regulating the rate of clock to correct the irregular 

 motion of the moon, and thus obtain absolutely fixed images on the photographic 

 plate. 



The advantage of large primary images in photography is low fully recog- 



