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XXXTII. 



ON AN IMPROVED EQUATORIAL TELESCOPE. By SIR 

 HOWARD GRUBB, F.R.S. 



[Read January 20, 1892.] 



The small equatorial carrying a 4-inch refractor, which I have the 

 pleasure to exhibit to-night before the Royal Dublin Society, pos- 

 sesses some novel features which I expect will be found useful, 

 more particularly for small instruments constructed for amateur 

 work. 



A large proportion of the trouble involved in working small 

 instruments consists in the necessity for the observer to leave his 

 observing chair continually for the purpose of reading the right 

 ascension and declination circles of the instrument. 



In large instruments this difficulty has of late years been met 

 by introducing contrivances by which the reading of one or some- 

 times both circles can be effected from the eye-end of the telescope. 

 This arrangement, however, would be almost as costly to attach to 

 small instruments as to large, particularly as it also involves some 

 illuminating arrangement to be brought simultaneously into action; 

 consequently the proportion of cost of such an arrangement would 

 be altogether excessive in the case of small instruments, although 

 inconsiderable in the case of large. I have, therefore, endeavoured 

 in this instrument to effect the same purpose by a different method. 

 1 have so constructed it that the ascension and declination circles 

 ure themselves at the eye-end of the telescope, and are caused to 

 revolve to their proper angular positions by certain gearing which 

 can best be described by the accompanying diagram. 



Firstly for the declination circle. A toothed circle //is attached 

 to the cross head of the polar axis ; into this toothed circle gears a 

 pinion g on a hollow shaft, the other end of which is supported in 

 a bearing h at the eye-end of the telescope. This hollow shaft 

 carries a pinion c which gears into a circle d, fitted to and strung 

 upon a portion of the casting which forms the eye-end of the tele- 



