1897 THE MICROSCOPE. 161 
minator to get more light, Several diaphrams made of 
blackened cardboard, were added and the instrument 
seemed complete. 
Prof. Abbe made an illuminator, which is the kind now 
made and sold with all microscopes here and abroad, but 
it is just the same as mine. 
The illuminator, which I made later was an achromatic 
prism. Unfortunately it is rather expensive as mounted 
in German silver, but is perfectly satisfactory. A prism 
is usally made of crown glass about two inches in the 
exterior sides, right angled, to reflect the light onto the 
stage. On one side it is plain and this is the immerging 
side for the light from the sun or source of illumination. 
The light is reflected on the bottom and issues from the 
opposite side, when it would be non-achromatic, were it 
not caught by ananchromatic lens, which is cemented to 
the prisms in the usual manner with Canada balsam. [I 
now would use gum thus for cementing the lens to the 
prism. The lens is plane on one side, that next to the 
prism, and convex on the outer side where the rays im- 
merge and are rendered converging on to the stage. 
Upper or lower side it matters not, though the under 
side is most common. The focus of the lens is five 
inches, but longer is sometimes convenient. Shorter is 
not necessary. With sucha prism the light can be used 
of a kerosene lamp and rendered bright enough to use 
with a1-12. It can be worked around to the horizontal 
or to the same plane of the stage. The advantages of 
such a prism are that the flint lens is enclosed between 
two crown surfaces, one the prism and the other the 
outer lens of the achromatic lens. It is thus kept clear 
from blows and although softer than the crown glass quite 
safe from injury. The apperture of the opening is great 
enough to render the light clear and bright. 
The other contrivance, which I have added to my mic- 
roscopic outfit, is a mode of rendering the ray of light 
