254 THE MICROSCOPICAL NEWS, 
glasses for objects between which and the cover-glass there is either 
a film of air, or of any fluid the refractive index of which is much 
different from that of glass. 
The use of glycerine as a mounting fluid for preparations stained 
with any of the aniline dyes is at best troublesome,* and sooner or 
later, to my experience, the staining runs, and the preparation is 
spoiled. Solutions of acetate of potash or chloride of calcium I 
have not found satisfactory ; the form, even of such resistant 
objects as bacteria, in some cases becoming materially altered by 
these reagents. With Canada balsam, even when dissolved in 
chloroform or turpentine, I have not found the preparations fade, 
as has sometimes been said to be the case, and as we should have 
expected ; nor, if they are sufficiently washed in alcohol and passed 
through oil of cloves, will they run; the risk, however, both of 
fading and running may be entirely obviated by using benzine as a 
solvent for the balsam, or by employing it undiluted and liquified 
by warmth. 
In examining this structure I have employed the th homo- 
geneous immersion of Messrs. Powell and Leyland, which having 
the very high numerical aperture of 1.38° gives, with admirable 
light and definition, an amplification of about 3400 diameters, with 
an eye-piece of ? in. focal length ; the barb, however, in a suitable 
preparation, may be readily seen and examined with a good ith 
objective. I have recognised it, even with lower powers, as the 
Toth, dependent however much upon the method of illumination 
employed : for, as is generally recognised, good illumination will 
show an object with a much lower power than is requisite in the 
ordinary way. The best means of this as yet available is the direct 
light of the flame of a paraffin lamp turned edgewise to the observer, 
whether with or without a substage condenser.t This was recom- 
mended by Dr. L. Beale thirty years ago, and is now again 
frequently adopted. Light reflected from any mirror is in some 
way inferior to direct light, and this not owing to the double 
reflecting surface of ordinary mirrors, for I have tried them silvered 
on the upper surface without any material advantage. 
* The method is, add an equal bulk of glycerine to the aqueous solution of 
the aniline dye used, stain somewhat more deeply than requisite, mount on slide 
with cover-glass in the staining fluid, which is to be gradually replaced as the 
water evaporates by plain glycerine. 
+ This may be found described and illustrated in Pritchard’s Microscopic 
Cabinet of over fifty years ago, and in Ledermuller’s Treatise of 1768.—ED. 
