270 
On a Method of estimating the Optical and 
thickness of the cover of any mounted object I possessed. I there- 
fore proceeded to find data for a ready estimation. 
First of all it was necessary to become possessed of an accurate 
gauge. Two methods are then necessary: to solve the problem. 
Actual thickness of various specimens should he tabulated by the 
gauge (I write on them with a diamond). I can strongly recom- 
mend the Swiss gauge sold by Messrs. Ehnhuus and Co., Frith 
Street, Soho. The instrument in my possession readily measures, 
with two jaws and a dial-plate and index, to the |th of a 
thousand. 
I find 16 divisions make seven thousandths, and a whole circle 
of 100 divisions, 45. 
The thinnest glass in my possession measures 24 thousandths. 
Having done this, the next step is to ascertain the interpreta- 
tion of the divided head of the “ fine screw adjustment ” : mine is 
divided into 20 parts, and by measurement the screw has 50 
threads to the inch. By means of a micrometer microscope, 
applied to the Powell and Lealand stand, I ascertained that — 
14 turns of milled head measured 65-25 divisions. 
11 microscope micrometer divisions measured -02 inch exactly. 
By rule of three — 
14 turns equal 
1 turn equals 
-Jjjth °f a turn equals 
2 divisions equal 
3 
4 
7 
14 
19 
JJ 
•1186 inch. 
•0085* „ 
•0004 „ 
•0008 „ 
•0012 „ 
•0017 „ 
•0029 „ 
•0059 „ 
•OOSO „ 
Now, in consequence of the lever of the fine adjustment acting in 
a curve tangentially, the values of the divisions on the milled head 
fine adjustment cannot be uniform. 
To ascertain these variations 
cally 
1 13 divisions 
13-3 „ 
125 „ 
12-5 „ 
13-0 „ 
examined a thick cover opti- 
Milled head fully out zero. 
Median position. 
. Fully home zero. 
. At one turn out. 
. At two turns out. 
Conclusion ; for the first six or seven turns of the milled head 
when fully out, the divisions may be considered as representing 
equal spaces in the axial line of fine adjustment. 
I now selected three different covers, and very carefully mea- 
* ‘84740 is the exact result. 
