71 
On the True Reading of Measurements with the Cobweb 
Micrometer. By Capt, J. Mitchell. 
(Communicated by F. C. S. Hoper, E.L.S.— Read March 14tli, 1866.) 
It is now some two years since I forwarded to the Editors of 
the Journal some remarks on the true zero position of the 
filaments in the cobweb micrometer. The paper does not 
appear to have reached the Editors. The mail steamer having 
been wrecked^ it was probably either destroyed or rendered 
illegible by the salt water. 
I have been so much engaged since that I had no time to 
return to the subject. In doing so now I have thought it 
would be preferable to request you to do me the favour to 
bring the subject before the Society. I hope your rules do 
not exclude communications from a non-member, at least 
from one residing abroad. 
The late Professor Quekett, in his ' Treatise on the Micro- 
scope/ 2nd ed._, p. 221, says, "the cobwebs should exactly 
coincide when the graduated head of the micrometer is at 
zero/^ and the same directions are repeated at p. 223. It 
appears from this that he did not take into consideration the 
thickness of the filaments, supposing^ perhaps, that they 
were too fine to afi'ect the measurements, which, however, is 
not the case. 
I assume that, in ascertaining the value of the divisions of a 
screw micrometer, we should endeavour to make the axes of 
the cobweb filaments coincide with the centre of the grooves 
ruled on the stage-micrometer, and that in measuring the 
distance between the strise of diatoms the same method would 
be pursued. In both these cases the true distance is that 
between the axes of the filaments ; and as these are supposed 
to coincide, when the micrometer head reads zero, the 
measurement will be correct. But the measurement of the 
interval between lines is neither the sole nor the chief use 
of a micrometer; on the contrary, the greater number 
of objects require to be placed between the filaments, and 
when this is the case the quantity shown by the micrometer 
head will be the diameter of the object plus twice the semi- 
diameter of the filaments, or, which is the same thing, the 
diameter of one filament. 
Now, with the same micrometer this excess is a constant 
quantity with all powers alike. With my micrometer a 
negative eye -piece, one by Powell and Lealand, with four 
filaments, amounts to two divisions of the micrometer head. 
