4 Transactions of the Royal Microscopical Society. 
In the examination of two objectives of equal power without 
some means of determining their relative apertures, it is quite pos- 
sible that an erroneous judgment may be formed. It may he that 
one glass will resolve a given diatom with ease which the other will 
not touch, and a hasty conclusion may be arrived at that the former 
is the better instrument ; but let each be examined for aperture, 
and the cause of the supposed superiority may at once be disclosed. 
The greater resolving power may be merely excess of aperture, and 
it may turn out that the better glass, both in its manufacture and 
for all practical purposes, may really he that which has failed on 
surface striation or dotted tests. 
It may he said generally that if two objectives differ widely in 
aperture they are incapable of comparison ; each may be excellent 
in its way, one in resolving power on surface markings, the other in 
what is now called “ penetration,” or depth of focus. But they 
cannot be compared. In emphasizing the word “ penetration,” 
attention is drawn to the change which has been made in the 
meaning attached to the word, which by some writers is used as 
the equivaleut of angular aperture. 
In absolute observations of every kind the element of “ personal 
equation ” exists, but it will be evident to all that in determining the 
difference of aperture of two objectives that element is necessarily 
eliminated. 
