A New Illuminating Ap;paratus for the Microscope. 81 
upper surface of the covering glass is almost entirely wanting. If 
the object be neither too transparent nor too opaque, and when 
other circumstances favour the effect in question, the pure positive 
images thus obtained may possess considerable brilhancy. In many 
cases— for instance, with diatoms mounted dry — the brilliancy is 
sujQ&cient, with good ordinary daylight, to allow perfectly well of 
an amplification of from 500 to 600, especially if the interval 
between the upper lens of the condenser and the slide is occupied 
by a drop of water. 
Should object-glasses having an angle of aperture sensibly 
exceeding 40° be employed with this kind of illumination, that 
angle will have to be reduced accordingly by stopping-off the zone 
of the periphery, partly because otherwise the field cannot be kept 
dark without reducing too much the effective surface of light in the 
condenser, and partly also because an excessive angle of divergence 
of the cone of rays forming the image interferes very materially 
with its perfection in the case of the great majority of objects. This 
stopping-off, which is indispensable when tolerably high powers are 
used, is effected by means of diaphragms with suitable aperture 
placed above the upper lens. The resolving power of the object- 
glass is of course reduced thereby to the degree corresponding to 
the free aperture that remains. 
I must leave it an open question whether and how far this class 
of illumination — which essentially is that of Wenham's paraboloid 
— may prove of any importance for scientific investigations. With 
the arrangement above described it, however, is not at any rate in 
the least liable to the objections raised against it relatively to the 
perfection of the image, and which are represented by Harting as 
inherent in its principle. On the contrary, with good object-glasses 
even with a considerable amplification — supposing the objects to be 
suitable — not only are very clear and distinct images obtained, but 
they are also frequently very characteristic ; and in consequence 
of their solid standing-out on the dark ground, may be said often 
to present decided advantages, at least as far as practical lectures 
are concerned. 
With regard to the use of the apparatus with transmitted light, 
it may be further observed that when very low powers are employed, 
if the available source of light is but of limited extent, no uniform 
illumination of the object is arrived at, owing to the image of the 
source of light in the focus of the condenser not being large enough 
to cover the whole extent of the object that is visible. One in this 
case obtains a field illuminated equally and with sufficient brilliancy 
by covering the plane mirror with a piece of white paper. If 
lamplight is to be employed for the observations, what answers best 
is placing as large a condensing lens as possible in a direct line be- 
tween the flame and mirror. The simplest method to follow is to 
