38 
Williams^ on the Martin Microscope. 
can be removed from the upper bar and fixed by the same 
screw to the longer arm. The mirror being removed,, this 
arm with the attached stage can be substituted for it^ and the 
ball and socket^ and other apparatus being applied as just 
mentioned^ a pleasing view of very large objects can be readily 
obtained. 
I must now call your attention to another peculiarity. 
Keeping the upper supporting bar in its present inverted po- 
sition^ we can apply another stage to this instrument. 
This is a piece of most elaborate workmanship. It con- 
sists of a frame to which a stage plate is fixed, and which 
is moveable in opposite directions by means of micrometer 
screws placed at right angles to each other. These micro- 
meter screws are provided with divided heads. The screw in 
each instance is one with fifty threads to the inch, con- 
sequently each turn moves the stage -^o^^ of an inch. The 
micrometer heads are divided into twenty parts_, hence every 
division is equal to y-^^th of an inch. It is easy to estimate 
by the eye a tenth part of one of these divisions should it be 
required J this would give the -ro,wo't^ of inch. There is a 
third micrometer on the side opposite to one of those already 
mentioned, this raises or depresses the stage to a similar 
amount. Hence it follows that as these micrometers are 
exactly alike, it is possible with this instrument to measure 
the length, breadth, and thickness of a minute object, certainly 
to -y-oVo^h, and, I believe, to -ro^-o o^b of an inch. Again, by 
means of a milled head, the inner ring of this stage is made 
to turn round, and thus we have a revolving stage of very 
accurate construction. There are no means apparent at pre- 
sent of applying this instrument to the measure of objects. 
I believe it was effected by placing in the focus of the upper 
lens of the eye-piece a disk of glass with fine lines drawn upon 
it at right angles to each other. Now, one end of the object 
to be measured, having been brought into the right position, 
and into contact with one of the lines on the glass by means 
of the revolving part of the stage, the micrometer screw was 
turned until the object had passed under the line, so as to 
bring the opposite end into contact with it, when the number 
of turns and parts of a turn gave the measure in thousandths 
of an inch. The breadth was measured in a similar manner. 
In measuring the thickness of an object the upper edge was 
possibly brought accurately into focus. The corresponding 
micrometer screw being now turned until the under edge was 
in like manner brought into focus, the measure of thickness 
would be read ofi* as in the former instances. This piece of 
apparatus appears to me to deserve particular attention, as it 
