6 
in(lei)endent of thickness of cover-glasses and also because any alteration in the 
distance of their lenses interferes with the perfection of their correction. Con- 
siderable variations in thickness of cover are best compensated for 
by slightly lengthening the body-tube for thinner covers, 
,, shortening ,, .. ., thicker ones. 
On page 21) it is shown in what manner the thickness of cover -glasses of 
a mounted specimen can be easily ascertained by means of our stands. 
The immersion liuid which we recommend for the homogeneous objectives 
is C e d a r - w 0 0 d oil (from Juniperus virginiana) which we have used from the 
first. We supply the same in a thickened condition, which not only does away 
with its inconvenient fluidity, but at the same time yields almost perfect identity 
of refractive index with that of the cover -glass. A bottle of this oil is supplied 
with each objective and may be procured from us at any future occasion (price 
M. 0.75 pr 1/2 oz bot). We expressly request, that no immersion 
fluids derived from other sources be used with our objectives, 
or at least none which have not been carefully tested as to their 
proper refractive power — as is done by us — because the use 
of unsuitable fluids implies considerable loss in the optical 
performance of the objective. 
As regards the relative merit of the homogeneous and noii- 
homogeneous (water etc.) immersion systems, the former claim superiority 
over the latter (focal length and aperture being equal) by reason of their eminently 
superior defining power and their greater insensibility as to variations of thickness 
of cover -glasses. Water immersion systems are only to be preferred in such cases 
where the nature of the objects does not admit of oil being used as immersion fluid. 
The mounts of all objectives are provided with the Eiiglisli standard 
thread of about 20 mm external diameter. In the series from A to J however, 
and also in DD when not fitted with correction adjustment, the lower part of the 
mount containing the lenses is made to unscrew from the adapter and may then 
be used with the narrow-gauge thread. 
The name of our firm is engraved on the mountings of all objectives, on 
the apochromatics also the aperture, focal length and tube length for which they 
are adjusted and on the ordinary glasses the letter by which they are designated. 
When ordering objectives which are intended for use with 
stands not made by us, it is in all cases, tvhere these are not noted 
for having the same screw-thread and internal diameter of tube, 
advisable to send the tube in question for the i^urpose of adapting 
the objectives and eye-jAeces. 
