_ 5 
All the others, particularly the ap ochrom atic series and 
also the homogeneous immersions, perform more or less defi- 
ciently on stands of English model, u n 1 e s s sp e c iall y a d j u ste d 
for these. 
In foreujn orders it should always he stated irhether the 
objectives are to be adjusted for the short (continental) or for 
the lomj (En(jlish) tube. 
Thickness of Cover. All objectives in lixed mounts are, unless otherwise 
ordered, corrected for a medium thickness of cover between 0.15 and 0.20 mm. 
In the higher series (from the apochromatic 8-mm and achro- 
matic D upwards) the thickness of cover corresponding to th e 
most perfect correction is indicated on the side of the mount by 
small figures (millimeter). It is, as a rule, sufficient for ordinary work 
with those objectives which we supply only in lixed mounts, to use covers of 
an estimated medium thickness. 
Homogeneous immersion ob- 
jectives are, within wide limits, inde- 
pendent of the thickness of cover. 
Correction Adjustment. The gra- 
duation and numbering on the correction 
collar, read off on the lixed index, indicates 
directly for each position of the collar 
the corresponding thickness of cover (in 
hundredths of a millimeter) which yields 
the best correction for that position. The 
correction for cover must be carefully ad- 
justed, particularly in the case of the apo- 
chromatics 4.0. rJ.O mm (dry) and 2.5 mm 
(water immersion) and the achromatic 
lenses F and J, failing which the ef- 
ficiency of the lenses wall be greatly 
diminished. 
The homogeneous immersion ob- 
jectives are only supplied in fixed 
m 0 u n t s beause, as already stated, their 
efficiency is within rather wide limits 
Figr. 1. 
The correction ring b serves to adjust the 
distance between the upper part d of tlie system 
and the lower part e which is attached to the 
mounting a. 
