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XI. 

 FLOATING EEFEACTING TELESCOPE. 



By SIR HOWARD GRUBB, F.R.S., Vice-President, Royal Dublin 



Society. 



(Plates VIL-IX.) 



[Eead, November 17 ; Keceived for Publication, November 20, 1903 ; 

 Published, February 10, 1904.] 



On the 21st February, 1894, I read a short paper before the 

 Royal Dublin Society (Scient. Proc. vm., pt. 3, p. 252), describing 

 a suggested form of equatorial suitable for reflecting telescopes of 

 the Newtonian type, in which the principle of flotation was carried 

 further than had hitherto been attempted. 



The object of proposing the flotation principle for these large 

 equatorials was to surmount the difficulty which is encountered 

 when the dimensions and weights of the moving parts become 

 excessive. The weights of the moving parts of such an instru- 

 ment increase about as the cube of the diameter of the object- 

 glass or mirror, whereas the bearing surfaces cannot be conve- 

 niently increased in a larger proportion than the square ; the 

 consequence is, that the weights on the bearings become excessive, 

 and great difficulties arise in reducing the friction sufficiently to 

 enable the ponderous masses of metal contained in the mounting 

 to be driven by the clock-work with that amount of steadiness 

 and uniformity which is absolutely necessary for modern astro- 

 nomical observations. 



Having lately been requested to submit designs for a refracting 

 telescope of 48 inches aperture, I was led to consider how far the 

 same flotation principle could be utilised in the case of refractors. 

 In order to effect this, I propose to adopt a very practical and 

 ingenious modification of the ordinary form of refracting telescope 

 suggested to me a few years ago by Professor Hale, Director of 

 the Yerkes Observatory in Chicago, where the largest existing 

 refractor is installed. He desired to have a refracting telescope 



SCIENT. PROC. R.D.S., VOL. X., PART II. M 



