16 
initial magnifying power, i. e. as if it were used as a simple 
lens without an eye-piece. They will be found of special service with 
immersion objectives, where great inconvenience is caused by having to change 
a lens already adjusted for another of lower power. 
The working eye -pieces for regular observation beginning with a magni- 
fication of 4 are likewise of entirely new construction and may be used with 
advantage even in the highest numbers. The eye- point in all lies so high 
above the eye-lens and the diameter of the lens itself is so large, that the usual 
inconveniences attending the use of eye-pieces of short focus are entirely obviated. 
Owing to the uniform position of the eye-points of the compensating eye- 
pieces any one of these may, without difficulty, be used with the usual drawing 
prisms, including, in particular, the Abbe Camera. The most appropriate for the 
purpose are naturally the lower powers 4 and 6. 
The numeration of tliese eye-pieces is carried out on the principle sug- 
gested by Prof. Abbe. The number which denotes how many times an eye- 
piece, when used with a given tube-length, increases the initial magnifying power 
of the objective, aifords the proper measure of the eye- piece magnification 
and, at the same time, furnishes the figures for rational numeration. On this 
basis the series of our compensating eye -pieces is arranged according to their 
magnifying powers, which are respectively 1, 2, 4, G, 8, 12, 18, (27), and these 
figures likewise serve as their designation. 
The magnification obtained by combining a compensating eye-piece with any 
apochromatic objective is arrived at directly by multiplying its number by the 
initial magnification of the objective as given in the preceding table. An 
objective of 3.0 mm focus for example yields in itself a magnification of 83.3 (cal- 
culated for a conventional distance of vision of 250 mm); eye-piece 12 therefore 
gives with this objective 12X83.3 = 1000. 
F or the continental and the English model microscopes 
two distinct series of compensating eye-pieces are made. The 
corresponding numbers in both series are of a different focal length according 
to the different lengths of the tubes. 
The eye -pieces 1 and 6 are only made for the continental, 
27 only for the English tube. 
An order for eye- pieces to be used on stands wldch are not 
of our make should be accompanied by an exact impression of 
the edge of tlte tube in sealinff-ivax or some other exact gatir/e of 
the internal diameter of the tube,' the best gauge, ivill, in all 
cases, be the tube itself. 
