19 
Aehromatie Objectives. 
Respecting the general character of these objectives we refer to the 
remarks on pages 2 and 4 — 6. 
To the list of achromatic lenses only one new system has been added. At 
the same time, however, most of the older types of achromatic lenses have been 
reconstructed with a view to more completely eliminate imperfections 
due to spherical and chromatic aberration than had been done 
formerly or — rather — than could have been done without the aid of the new 
glasses. With the dry lenses DD, E and F this improvement becomes obvious 
by the clearer appearance of the image and the increased sensibility of 
the lenses with regard to differences in the thickness of the 
cover - glass and length of tube. 
To the achromatic lenses enumerated in our last catalogue we have recently 
added a water immersion lens of great focal length but having a relatively small 
ajierture, viz Objective D*. It is calculated to facilitate the examination at 
medium magnitications (200 to 500 diameters) of living zoophytological objects 
Hoating in water troughs. Its large focal distance gives to the microscopist 
a relatively wide range in following the motions of these living objects. The 
objective being constructed on the water -immersion i)rinciple, i. e. there being 
always a layer of water interposed between the objective and the cover- glass, 
the correction of the aberration will not be affected by focussing at various 
depths of the water-cell, as to a diminution of the stratum of water under the 
cover-glass always corresponds an increase of the stratum of the water above 
the cover -glass, and vice versa. This manner of using the lens will, con- 
sequently, not interfere with the quality of the image. The relatively small 
aperture is conditional to the great focal distance; for that kind of observation, 
however, for which the lens is mainly designed this will, owing to the increased 
depth, constitute rather an advantage than a disadvantage. 
The system maybe used with or without cover-glass and gives equally 
good images both with fresh and sea water. 
