123 
The President's Address. By E. M. Nelson. 
Now we come to Mr. Grayson’s, whicli are said to be on a new 
formula (not published), and which are quite permanent. There can he 
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 he has so kindly presented 
to the Society is the one which contains lOOths and lOOOths of an 
inch, and lOths and lOOths of a millimetre. 1 have very carefully 
micrometrically measured these rulings, with an oil-immersion 1/8 
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 difference 
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 1/130,000 in., yet you can 
measure inequalities in larger objects, certainly up to 1/500,000 in., 
and probably less, as half a division 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 fi, the maximum and minimum 
errors are +’097 and —*072 //, respectively. With regard to the 
ratio between the inch and millimetre scales, we have 
4502*5 
177*38 
25*3821. 
Now the value of the inch, when compared with the metre, f both 
being at a temperature of 62° 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 micrometer, 
an old one was measured, and the difference between the maximum 
and minimum values was found to be no less than 1/16,271 in. — a 
very different result. 
When Mr. Grayson’s millimetre rulings were compared with 
those by Mr. Kogers, it was found that they came out very favour- 
ably. Unfortunately, Mr. Rogers’ scale is mounted dry, and has 
sweated considerably, consequently the measurements could not be 
carried to such a fine point as before. Mr. Grayson’s rulings being 
* Journ. R.M S , 1890, p. 510, and Carpenter on ‘The Microscope,’ 7th ed. , 
p. 229, fig. 177. 
t Results as obtained by measurements made by order of the Board of Trade in 
1896. 
