60 Maltwood^ Oil a Finder. - 
4tli. The different parts of the scale should be so distin- 
guished as to determine at once, on looking through the 
microscope^, what part is in the field. 
On reading the account of the dififerent methods which 
had been brought before the Microscopical Society, that 
proposed by Mr, Farrants appeared to me to answer to more 
of these requirements than any other, but this was still de- 
ficient in the last particular, viz., that of being able to de- 
termine, on looking at it through the microscope, what part 
of the scale is in the field. The attempts I have since made 
have been to supply this deficiency. 
The first idea I attempted to carry out was that of a scale 
divided into fiftieths of an inch ruled with coloured lines, 
the centre ones being black, and the others of four different 
colours in regular succession, every fifth line being a double 
one. By this method it was never necessary to pass more 
than two fiftieths of an inch across the field to the nearest 
double line in order to ascertain your exact position on the 
scale. The first one I made I ruled on paper gummed on 
a glass slide, but the difficulty of ruling it, with any degree 
of correctness for use under a high power, seemed an obstacle 
to its becoming a plan of general utility. 
I afterwards thought of the possibility of photographing 
some kind of scale for the purpose, but not seeing at the 
time any good method for distinguishing the different parts 
of it, I did not attempt anything in this way, and was again 
engaged in endeavouring to carry out my first idea of the 
coloured lines upon a transparent section of ivory cemented 
on glass, when a friend, with whom I had talked over my 
different plans, suggested to me the possibility of distinguish- 
ing the lines in a photographed scale by using figures. This 
at once struck me as being very feasible, and, after a little 
consideration, I felt sure that a scale might be photographed, 
not merely with figures indicating the number of the lines, 
but that, by employing two sets of figures, I might have a 
finder in which every one of the 2500 spaces in the square 
inch would have both its latitude and longitude recorded. 
By simply writing down the figures that presented themselves 
in the field, when this scale was made to take the place of 
the object, you would at once have its precise position 
registered. I succeeded in carrying out this plan in the 
following way : 
I had a scale, ten inches square, divided into fiftieths, each 
space being one fifth of an inch square, and wrote in the 
upper part of each space the figures representing the latitude. 
