Tue Microscope. 113 
is very distinctly appreciable, so that the high power work of 
the future will almost necessarily be done with these glasses. 
The objectives in question are both }in. The special point 
in their construction is that they are made of new kinds of op- 
tical glass, which Prof. Abbe and Dr. Shott have been working 
for the last five years to perfect. The objectives are composed 
of ten single lenses, combined to five separate lenses, with a 
single front lens. Their working distance is 0.25 mm., and in 
order to secure this the aperture is limited to 1.40 N. A. With 
the length of tube engraved on the setting (taken from the 
nose-piece to the eye-lens), the objectives have their best correc- 
tion for a cover-glass of 0.16-0.18 mm. Much thinner covers 
require a lengtheniug of the tube by 10-25 mm. further. They 
are very sensitive in regard to length of tube, and the change 
in this length is the simplest, and in fact the best, means for 
slight corrections for different covers—the reason being that a 
change of that kind does not alter the proper balance of the var- 
ious corrections (spherical, chromatic and sphero-chromatic), 
whilst an alteration in the distance of the lenses of the objec- 
tive from one another, as is done by a screw-collar, does disturb 
that balance to the injury of the performance of the objective. 
It may be possible to find a formula which will be less sensitive 
in regard to this question of correction, but until it is found, 
Dr. Zeiss, by whom the objectives are made, will not supply 
any with correction-collars, so as to convert a good objective 
into a medium one for the sake of a non-essential convenience 
only. 
A novel point in counection with the objective is that its 
performance is improved by the use of special eye-pieces, of 
which two are supplied, of 25 mm. and 15 mm. focal length. 
Their function is to compensate for certain aberrations outside 
the axis, which cannot be compensated for in the objective. 
With these eye-pieces, particularly with that of 25 mm. focal 
length, the field of view is surprisingly uniform. 
Of the ten lenses of which the objective is composed, two 
only are of siliceous glass, the other eight being made of borates 
and phosphates. The crown and flint glass now used by optic- 
ians does not contain (as essential components) more than six 
