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XV. 
SOME NEW FORMS OF GEODETICAL INSTRUMENTS. 
By SIR HOWARD GRUBB, F.R.S., Vice-President Royal Dublin Society. 
(Pratt XXXTYV.) 
[Read Drecemper 18, 1901. |] 
Tae 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. vul., series I1., 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- eS 
ing or re-focussing of the eyes such as —_ ———————— rN 
is the case when using the ordinary A— se 
sights. 
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Description.—The original concep- Be, th 
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. 
TRANS. ROY. DUB. SOC., N.S., VOL. VII., PART XV. 2K 
