PROCEEDINGS OF SECTION A. 189 
attention must be given to it. This importance is twofold— 
fir-t the medium, secondly the rulings. 
“Mr. Grayson’s are said to be on a new formula (not pub- 
lished), and are quite permanent. There can be no doubt that 
both the diatoms and the rulings mounted in it show up in a 
very superior manner. Now, with regard to the micrometric 
rulings, the most important of the slides is the one which con- 
tains 100ths and 1000ths of an inch, and 10ths and 100ths of a 
millimetre. I have carefully micrometrically measured these 
rulings with an oil immersion, ¢ of 1.43 N.A., and a very fine 
micrometer eye piece by Powell. Taking the ruling of .001 
in. first, we find that the mean for the ten ruled spaces gives a 
micrometric value of 450.25 divisions of the screw head. The 
difference between this mean and the widest space was + 1.25 
divisions, and the least was — 1.25. Now, as the value of one 
division is 1/450,250 in., the maximum and minimum error in 
each case is 1/360,200 in. This may seem to some ridiculous ; 
but allow me to state that a half of a single division made a differ- 
ence that could be perceived. Therefore, it appears that. while 
with the best microscopical appliances of the present day 
you cannot separate a quantity less than, say, 1/120,000 to 
1/30,000 in., yet you can measure inequalities in larger objects, 
certainly up to 1/500.000 in., and probably less, as half a divi- 
sion of my micrometer meant under the conditions in which 
it was used 1/900,000 in. 
“Now with regard to the millimetre rulings, we find that they 
are remarkably accurate, the maximum differences from a mean 
of 354.7 for .02 mm., being only + 1.72 and — 1.27. As the 
value for one micrometer division is .05637 »., the maximum 
and minimum errors are + .097 and — .072 w. respectively. 
With regard to the ratio between the inch and millimetre scales 
we have— 
Now the value of the inch with the metre, both being at a 
temperature of 62 dee. F., is 25.3999779; we see, therefore 
that Grayson’s has slightly too small a ratio. 
“To illustrate the advance we have in Mr. Grayson’s micro- 
meter, an old one was measured, and the difference between 
inaximum and minimum values was found to be no less than 
1/16,270 in.—a very difierent result. 
“When Mr. Grayson’s millimetre rulings were compared with 
those by Mr. Rogers, it was found they came out very favour- 
ably. Unfertunately, Mr. Rogers’ scale is mounted dry, and has 
sweated considerably, consequently the measurements could not 
be carried to so fine a point as before. Mr. Grayson’s rulings 
