242 Tur Microscope. 
lens of its angle I ever saw. By central lamp light, it resolves 
Moller’s No. 15 (Pleu. fasciola) into dots; while with oblique light 
without condenser or hemispherical lens—the resolution of Am. 
pellucida is easy. The crucial test, however, was a silver-plated 
valve of Am. pellucida, shown with the vertical illuminator. By 
this method the resolution into dots was easy when the light was 
properly managed, and by slight changes either the transverse or 
longitudinal lines could be brought out. Selecting a certain line 
or row of dots, and using a { inch eye-piece, both Mr. C. E. Barr, 
of this city, and myself counted the dots and got them the same. 
I am well aware that many persons doubt the resolution of 
this diatom into dots, claiming that the apparent dots are caused 
by longitudinal diffraction lines, crossing the transverse real lines. 
That this is sometimes the case—especially in photographs—I do 
not doubt; but when caused in this way the number of dots in 
any particular row, may be made to chinge by a very slight change 
in the focal adjustment, while in the case of a true resolution such 
a condition is not seen unless the objective be thrown considerably 
out of proper correction. 
When the fine performance of this objective is considered in 
relation to its very long working distance, it will be seen that it is 
a glass of more than ordinary utility, for it is a well known fact 
that good corrections and long working distance are too a certain 
extent antagonistic; and, although this will not apply with as 
much force to homogeneous-immersions as to water immersions or 
dry lines, still it is a matter of considerable importance. 
I have seen lenses of far greater relative working distance, but 
in such cases, even when having larger apertures, the definition has 
not been equal to those of shorter working distance and narrower 
angles. 
A NEW SLIDE CABINET. 
EUGENE PINCKNEY. 
DEMAND exists for a Slide Cabinet that shall be neat com- 
pact, accessible, strong, inexpensive and easily constructed. 
I think I have such a pattern It consists of a box or case con- 
taining drawers, with or without a front door. Mine is 14 x 
21 x 23 inches, has twenty-seven drawers and holds 1,500 slides 
on the flat. The case can be made of any size, of hard or soft 
