THE Microscope. gf 3 
demand for these high powers is gradually decreasing. By high 
powers I mean all those over a 1-16. Although there is per- 
haps much to bedesired in the improvement of objectives, we 
may still consider our present state quite an advanced one. 
Although the present theoretical limit of visibility is fixed at 
146,528 lines to the inch, we need not be deterred from attempt- 
ing to pass this point. The limit which was accepted some 
years ago as the true one, although considerably lower, was 
quietly ignored as the angular aperture in objectives increased. 
It is only a few years ago that the majority of microscopists re- 
fused to believe that A. Pellucida, which has about 100,000 
lines to the inch, could be resolved, and now it is the work 
of beginners to do so. 
But supposing 146.528 lines to be the limit, it is evident 
that a 4 or » objective with a 4-inch eye-piece, is of amply 
sufficient magnifying power to make the lines visible to the eye, 
and there is therefore no need of using more. It is a good rule 
to follow, under all circumstances, not to use a greater power 
than is necessary to comfortably do the required work. Many 
people are unmindful of the fact that the amount of light which 
enters a room through a window is considerable more than that 
which comes through a key hole. The loss of light increases 
with each additional power, and it is perfectly natural when we 
consider that the openings in which the lenses are mounted de- 
crease in the same ratio. 
Maynifying powers may be varied by means of the draw 
tube, but is seldom used, as it destroys the standard to which 
the objectives were adjusted. In speaking of a standard, I am 
forced to ask, Have we one? It is safe to say that none exists; 
and it is strange that after these many years which have been 
devoted to the improvement of the microscope, and where such 
marked improvements have been made, so little attention 
should have been paid to this important matter, and such a lim- 
ited knowledge of the subject should exist. 
It is said that 10 inches make the standard, but what con- 
stitutes the 10 inches? No book states whether it should be the 
length of the tube, or whether the distance is from the front of 
the objective to the eye-piece, or from the back of it to the eye- 
piece. The matter is receiving the attention of mathematic- 
