TRANSACTIONS. 
On a New Method 0/ Micrometry. By Wm. Sydney 
GiBBONs^, Melbourne;, Victoria, 
(Read Nov. 24tli, 1858.) 
Situated^ as I am^ at so great a distance from the great 
centres of science^ and deeming myself, in comparison with 
those whom I have the honour to address, as yet but a 
student, I feel some diffidence in submitting the results of 
my labours. For many matters which seem to me important 
may be considered trivial by those older and better skilled 
than myself, and what is really new to me may have been 
anticipated in the old country. Nevertheless, I desire to 
make ])rogress myself, and to be useful, as far as I can, to 
others. 
As the object of this paper is to introduce a micrometric 
contrivance of my own which I believe to be new, and have 
found very effective, rather than to give a history of micro- 
metry, which would be tedious and out of place, I shall not 
occupy time or space by describing at length the other in- 
struments employed for accomplishing the same end. But 
it will be necessary to compare some of the modes, for a 
twofold purpose : first, to explain more clearly the nature of 
my supposed improvement; and secondly, to show that, 
although it has been nearly approached, it is in fact original 
and new. I shall, then, be as brief as possible. The micro- 
meters already in use may be considered as of four kinds. 
1. Scales applied to the object, the divisions being arbitrary 
and requiring proportional calculation ; of this kind were 
the rough measurements of Leeuwenhoek, and the more 
perfect scales of Lister, who placed on the stage a slip of 
ruled glass, the value of whose divisions was known, and viewed 
them in comparison with a micrometer of the second class, 
in order to ascertain the value of the divisions in the latter. 
2. Scales applied to the eyepiece, so as to coincide with the 
magnified image of the object as projected into the body of the 
instrument by the objective, and to be viewed with it, so that the 
two images of scale and object were presented together to the 
eye. Of this class are Jackson^ s, Bamsden^s, and the cobweb 
VOL. VII. e 
