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XV. 



SOME NEW FORMS OF GEODETIOAL INSTRUMENTS. 

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



(Plate XXXIV.) 

 [Eead Decembek 18, 1901.] 



The principles involved in the gun-sighting apparatus described in a paper of 

 mine read before the Royal Dublin Society, March 20th, 1901, and published in 

 the Scientific Transactions, vol. vii., series ir., are applicable to all instruments 

 used for observing the direction or bearings of distant objects, more especially 

 for surveying and geodetical instruments of almost every variety. 



In the present paper I propose to describe how these principles can be 

 utilized in the case of some of the simpler and best known surveying instruments, 

 such as the Plane Table or Graphometer, the Level, the Prismatic Compass, and 

 the Clinometer. 



Referring for a moment to the above 

 paper on the gun-sighting apparatus, it 

 will be observed that by a simple optical 

 contrivance a virtual or "ghost" image 

 of a cross, or any similar device, is seen 

 as if projected on to the object aimed 

 at. This image is formed on the same 

 plane as the object itself; there is there- 

 fore no parallax and no necessity for any 

 backsight, and the aiming of the weapon 

 is effected by the simple superposition 

 of the cross and object, both of which 

 can be seen distinctly without any strain- 

 ing or re-focussing of the eyes such as "_~_~|— — — ^^"^^^^i J-l^-- r-^.~ ".jl J ""j^ 

 is the case when using the ordinary A 

 sights. 



Description. — The original concep- p^^ I 



tion of this sight, as described in the 



paper above referred to, is represented in fig. 1, The object to be aimed at is 

 viewed through a short piece of tube {AB) preferably square, open at both ends, 

 in which is mounted, at an angle of 45^, a plate or plates of parallel glass, similar 

 to those used in sextants. 



TEANS. EOY. DUB. SOC, N.S., TOL. TU., PiET XV. 2 K 



