Maltwood^ on a Finder. 
61 
taking the top line as the equator, and beginning with the 
top row, all of which I numbered 1 ; in the second row from 
the top I wrote the figure 2 in the upper part of each space, 
and so on to the bottom. I then filled in the figures in the 
lower part of each space to denote the longitude, beginning 
with the first row on the right-hand side, all of which, from 
the top to the bottom, I numbered 1 ; the second row from the 
right I numbered 2 ; and so on to the last row on the left, 
which would be of course 50. 
This scale I afterwards had photographed, taking great 
care that the negative produced in the camera should be 
exactly one inch square, which was done by marking a square 
inch upon the focussing glass, and making the image of the 
scale fill up the space. Having obtained a negative of the 
right dimensions, I had a positive printed from it. This now 
showed the scale in the proper way for reading it under the 
microscope, the figures being all inverted when the printed 
surface was placed upwards with the space numbered \ at the 
left bottom corner. 
I then cut the glass on which the positive was taken, as 
near to the required size as possible, and afterwards ground 
down the edges, until I brought the centre of the scale — 
when placed under a quarter objective — to coincidence with 
the centre of the cross ruled on one of Mr. Jackson's glass 
standards. 
The plan I should in future adopt, to get the scale cor- 
rectly centred, would be, to cut the negative to the required 
size, and then to grind the edges till the centre of the scale is 
brought to one and a half inch from the right end (looking 
at it from the surface), and half an inch from the bottom of 
the slide. The positives may be printed from this negative 
on slips of glass three inches by one ; and by taking care that 
the corner of the slide from which the measurement is taken 
be always placed in the same corner of the printing frame, 
and that both the negative and positive are kept well up 
together to that corner, it will not only ensure perfect coin- 
cidence in any number of positives that may be printed from 
this negative, but it will also materially diminish their ex- 
pense, as the positive will require nothing further than to 
have a covering of thin glass cemented over it with Canada 
balsam. 
Nothing can be more simple than the use of this scale. 
Whenever I find an object I wish to register, I fix a lateral 
stop at the left end of the slide, and after bringing the object 
to the centre of the field, remove it and place the finder on 
the stage, taking care that the left end is against the stop. 
