248 MICROSCOPY. 
be seen in different aggregations where the line has not been per- 
fectly filled in, and if at the rate of two atoms of black in the 
square of the line, the five thousand millionth becomes the ten 
thousand millionth; if at the rate enty atoms of black, the 
size of the atom is the one hundred thousand millionth of an inch. 
now come to the most important and, to my mind, the most 
interesting part of the subject, which deals with the tests unblack- 
ened. For this purpose I must go back to the square of the line 
forming the letter as the five billion three hundred and forty-nine 
million seven hundred and twenty thousandth of an inch that, 
reduced to its square root, gives seventy-three thousand plus of 
an inch linear as the breadth of the line. 
mount the same series of slides in the way that Monsieur 
Nobert mounts his justly celebrated tests — without black—and 
helping to afford the power of determining at what breadt | un- 
blackened lines become invisible, even when aided by the micro- 
scopes of the present day. In this instance the seventy-three 
line. 
the same test unblackened. More than that, although I know 
the exact spot that it occupies, and mark the spot with an In- 
dian ink ring before it leaves the machine in which it is engraved, 
I have never (perhaps because of irritable temperament) been able 
to discover not merely the line, but the aggregation of lines form- 
ing the two hundred and twenty-seven letters of the very small 
tests, although they become perfectly distinct when black.” 
As a test of distortion, Mr. Webb rules fine black lines upon 
two pieces of glass, and places one upon the stage and the other 
upon the diaphragm in the ocular. 
_ AnmarcuLeEs ix Burreruink.—The “Pacific Medical Journal” 
: believes, with much reason, that the abundance of animalcules dis- 
