276 NATURE 
[JuLy 17, 1902 
SOME NEW FORMS OF GEODETICAL 
INSTRUMENTS. 
HE optical principles involved 
in gun-sighting 
apparatus, described in the issue of NATURE for | 
January 9, 1902 (p. 226, vol. Ixv.), have been further 
developed by Sir Howard Grubb, F.R.S., and applied to 
some new forms of geodetical instruments. In the gun- 
sighting apparatus alluded to,a virtual image of an 
illuminated cross is optically projected on to the object 
aimed at, and both the cross and the object are easily 
seen without any refocussing or straining of the eyes. In 
the case of the gunsights and also the present instru- 
ments, light traverses a plate of glass coated with a very — 
thin film of galena; by this means reflection of light 
from the surface of the glass is greatly increased, while 
but little transmitted light is shut off. The process of 
depositing galena is due in the first instance to Prof. 
J. Emerson Reynolds, F.R.S.; it is described in the 
Proc. Chem. Soc. for 1884, under the heading ‘“ The 
Synthesis of Galena by means of Thiocarbamide.” 
The process has been modified by Mr. G. Rudolf 
Grubb and applied with great success to some new forms 
of surveying instruments. 
Fic. 1. 
been designed to take the place of the standard instru- | 
ments of the engineer, namely the level and the theo- 
dolite, but to place in the hands of comparatively 
inexperienced observers, a ready means of making a 
rapid survey with an accuracy as great as can be attained 
in plotting a survey on paper. In the case of ordinary 
surveying, for example, in road making and in the con- 
veyancing of property, the accuracy of the survey is 
limited by the degree of precision with which it can be 
actually plotted on paper with a pencil giving fine lines. 
When the theodolite is used, the readings are first entered 
in the field book and then afterwards plotted on paper, 
the angles being set off with a protractor. 
the new instrument, the survey 1s continuously plotted as 
the instrument is being used. In Fig. 1 the new form 
of plane table is shown. The central pillar, through which 
the successive bearings are taken, is shown in section in 
Fig. 2 ; it is mounted on a triangular base, or set square, 
which can be rotated about a point situated in the centre 
of the paper on the plane table. The instrument is used 
thus. The sight tube is rotated until its fixed line coincides 
with a given object, a line is then ruled, it is again 
moved through some angle till the line coincides with a 
second fixed object, and another line is ruled along the 
NO. 1707, VOL. 66] 
These instruments have not | 
By means of 
edge of the set square, the process being repeated until 
the position of the last fixed object is recorded. Then 
the whole plane table is moved to a fresh station at a 
measured distance from the first station, and similar 
observations are made on the same fixed objects ; the 
intersections of the two sets of bearings give the points 
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required for the survey. In the case of the survey of a 
small area, the instrument is not shifted to a new station, 
but the distances corresponding to the ruled lines are 
determined by reading the number of divisions which 
appear in the field of the instrument between two marks 
Fic. 3 
of known distance apart on a staff held at a fixed point. 
The instrument thus becomes a telemeter, and by means 
of a suitable scale the distance along any given direction 
is found and marked on the paper. The actual method 
| of using the instrument is as follows :—The staff man 
