In both cases two eyepieces are employed, one to view each separate web.
- Diagram of the diaphragm in eyepieces of " the micrometer used for measuring the plates of the Astrographic Catalogue.
Small German periscopes were usually 1 metre or 2 a metre in length and had two eyepieces giving magnifications 10 and 15 diameters.
Here they pass through the lenses and prism shown into one of the eyepieces, F.
There are three eyepieces which are mounted on a revolving sleeve in such a way that any one of them can be quickly brought into use, to give the magnification suitable to the height of the mast.
The two most customary eyepieces consist in two simp:e planoconvex lenses, whose distance one from the other is equal to half the sum of the two focal lengths.
In both eyepieces micrometers or cross-wires are used for measuring in the plane of the real FIG.
These eyepieces are intentionally provided with a different chromatic magnification, which however is in opposition to that originating in the objective.
These eyepieces are very convenient in use, for when they are changed the lower focus always falls in about the same plane.
In German and French microscopes the optical length of the tube, when apochromats and compensation-eyepieces are used, is 180 mm.
As powerful achromatic objectives show differences of chromatic magnification in the same way as apochromats, compensation eyepieces can be used in combination with these objectives.
The first of such eyepieces was proposed by R.
The system of Porro prisms employed affords a convenient method of adapting the ends of the eyepieces to the interpupillary distance.