294 
Proceedings of the Royal Irish Academij 
XXXIII. — Ois A NEW Form of GomoMEXEE. By Joseph P. O'Eeilly, 
C. E., Professor of Mining and Mineralogy, Poyal College of 
Science. "With Plate XX. (Science). 
[Read June 24tli, 1872]. 
The principle involved in the construction of the Goniometer now 
submitted to the Academy, applies rather to instruments intended for 
the measurement of angles in general, than to those which are specially 
destined to the measurement of the angles of crystals. This principle 
is, the substitution of a straight edge or divided scale in the determina- 
tion of angular measurements, for the divided circle usually employed. 
Now as a straight edge can be more easily and more accurately di- 
vided than a circular one, and as length of edge is, in this case, the 
sole condition as regards dimension, requisite for the attainment of mea- 
suring power, it is evident that such a substitution must at once both 
simplify and cheapen the instrument, while the bulk being considerably 
reduced, and the form rendered very convenient, the instrument can be 
rendered quite portable and safe for the purposes of the field Geologist 
and Mineralogist. This new form may therefore be considered as pre- 
tending to simplicity, reduced bulk, and accuracy, and consequently to 
cheapness and usefulness. Those latter considerations are most impor- 
tant from the point of view of education. Students, as a rule, are not suf- 
ficiently well acquainted with the diff'erent instruments brought under 
their notice in scientific lectures, nor can they well become acquainted 
with many of them on account of their costliness, bulk, and delicacy 
of arrangement ; but with even the more ordinary instruments, there 
is generally wanting that degree of familiarity which induces the use 
of them for observation, and that knowledge of their relative degrees of 
utility, which is really the fundamental point of all sound observation. 
To cheapen an instrument is therefore to put it more generally within 
the reach of students. To simplify it without lessening its accuracy is to 
encourage them to a more frequent use of it, and thus really to induce 
them to study science in a practical manner. Such was my principal 
object in attempting to improve on the ordinary Wollaston Goniometer. 
For cabinet-work no instrument can be more perfect than this, and few 
have rendei'ed greater services in their way. But its bulk renders it 
unsuited for the purposes of the field student, while it is really in field 
work that the easy and rapid determination of the special charac- 
teristics of minerals becomes of great value ; and of those none can be 
more important as far as regards crystalline minerals, than the deter- 
mination of the angles. Is it possible to attain the results given by 
the WoUaston Goniometer by means of an instrument equally precise, 
but much more simple, and more reduced in bulk ? Such was really 
the problem which I undertook to solve. IN'ow whatever may be the 
form of the instrument to be employed, there must be necessarily 
an axis of rotation on which to adapt the crystal requiring measure- 
