248 THe Microscope. 
thicker than is required by the angular aperture. This is done 
mostly to obtain a higher degree of correction of the spherical 
aberration; the increased thickness of the front lens working 
the same as an immersion medium. Therefore the optical 
performance, especially of the resolving power of a dry working 
or water immersion objective, is to be regarded as directly pro- 
portionate, not only to its angular aperture but also to its 
relative working distance. Hence, if two objectives are equal 
in focal length as well as angular aperture but different in 
working distance only, the one with the shorter working distance 
ought to do correspondingly more than the other. 
AN UNUSUAL INFUSORIAN OF THE FAMILY VORTI- 
CELLID 2. 
D. S. KELLICOTT, BUFFALO, N. Y. 
CAJAQUADA CREEK, after flowing westward ten or 
. twelve miles through rich farming lands and the north-east 
part of the city of Buffalo, passes through Forest Lawn ceme- 
tery into Park Lake; about a mile below the lake it enters the 
Erie canal at Black Rock. Like most streams through popu- 
lous districts, it has more or less the character of an open sewer. 
Park Lake, fed by this stream, is an artificial pond in the beau- 
tiful and extensive public park of Buffalo. By reflection, one 
may readily perceive that its water is not quite limpid and 
wholesome, not as nearly so as it should be in a boating resort, 
and that the combination giving sewer and cemetery contamin- 
ation to the park water is one not to be recommended. The 
creek below the park is wide and deeper than it is above the 
same; the construction of the canal having changed its former 
outlet into the Niagara River and raised considerably its level, 
so that it now fills its ancient excavation in the drift-clay. 
The numerous stumps of trees still standing in the water attest 
the changes of level made nearly seventy years since by “ Clin- 
ton’s Ditch.” 
As the contaminated water of the creek spreads out in the 
broader, quiet lake and slowly moving stream, its organic mat- 
ter is rapidly separated by settling and decomposed by bacteria, 
