18 
^Projection Eye-pieees. 
These are used for projecting the image formed by the objective on a screen 
for demonstrating purposes or upon a photographic plate. They consist of a 
convex lens and a compound system, which is most carefully corrected both 
spherically and chromatically after the principle of apochromatic lenses, and is 
entirely free from secondary chromatic aberration and from difference of 
focus between the visual and chemical rays. A diaphragm is placed 
between the lenses for limiting the field, and the compound lens can be made 
to approach or recede from this diaphragm. The cap of the projection eye-piece 
forms a diaphragm by which internal reflexion in the body-tube is entirely ob- 
viated. The aperture of this diaphragm is made to correspond with the greatest 
aperture of the apochromatic lenses. 
The projection eye-pieces are specially corrected for our apochromatic lenses 
on the principle of the compensating series, but may, nevertheless, be 
advantageously employed with ordinary achromatic lenses of 
large aperture. 
The designation of these eye -pieces corresponds, as in the case of the 
other compensating eye -pieces, to those amplifications which they would yield 
(magnifications being measured after the same principle as for the other compen- 
sating eye-pieces) il used for ocular observation. 
The magnifications are 2 and 4 for the l()0-mm tube, 3 and 6 for the 
2r)0-mm (10") tube. 
We do not construct projection oculars yielding higher amplifications beyond 
those indicated as these would not possess a,ny practical advantage over the ordinary 
compensating eye-pieces, which can be used when higher amplifications are required. 
Tlie magnification for any distance of image from the eye-piece is obtained, 
by dividing this distance, expressed in millimeters, by the focal length of the 
objective in use and multiplying the result by the number of the projection eye- 
piece employed. Thus the ocjective of i) mm gives with the projection eye-piece 2 
an image magnified lOTK) times at a distance of 150 cm. for 
This rule holds good, strictly speaking, for long distances only; for short distances 
the figure so obtained is in excess of the true value. 
The image distance may be reduced in the case of 2 and 3 to about 
4( KD mm and with 4 and G to about 250 mm (reckoning from the eye-piece) ; it 
may, however, be increased to any desired length. 
For further details see "Specialkatalog fiir Mikrophotographie" (German). 
Price of the Projection Eye-pieces: 40 Marks each. 
