JULIUS RHEIXBERG ON STEREOSCOPIC EFFECT AND A 



both eye pictures, and that the question as to the right or left 

 half of the objective being used to produce the picture for either 

 eye ifi l>ti; a secondary matter dependent on this, led Abbe to 

 invent his binocular eye-piece, or "stereoscopic ocular," as it is 

 usually called, Willi this all the effects obtainable with other 

 forms of binocular microscopes can be produced, and it lends 

 itself to further effects besides. Abbe argued that, so long as we 

 can obtain the necessary and correct parallactic displacements, 

 it does not matter how or where they are produced. Since the 

 Etamsden circle (or exit pupil) above the eye-piece of the micro- 

 sco] e is an image of the objective aperture, and forms a common 

 section of the light pencils emanating from all points of the 

 objective, cutting off a section of the pencils there amounts to 

 the same thing as cutting them off just above the objective. So 

 he arranges that two identical images are formed in the two 

 eye pieces by allowing the whole of the light to pass a partially 

 reflecting surface in the microscope tube, and then the two inner 

 halves of the Ramsden circles are blocked out by adjustable 

 eye-caps, having a semicircular aperture as shown in Fig. 7.* The 

 resulting images show precisely the same differences as if the 

 separate halves of the objective had been used. If, now, the 

 eye-caps are rotated so as to block out the outer halves of 

 the Ramsden circles, and only allow the light from the inner 

 halves to reach the eye, the contrary effect is produced. The 

 objects appear turned inside out— they are seen u pseudoscopi- 

 cally" instead of " orthoscopically," to use the recognised nomen- 

 clature. We may, if we wish, remove the eye-caps altogether. 

 Nearly all the stereoscopic effect, except that which we obtain 

 from either picture viewed monocularly, then vanishes nearly— but 

 not quite, because, as already mentioned, the mere fact of using 

 both eyes together appears to strengthen any impression of 

 plasticity. From the foregoing it will be seen that no better 

 evidence of the true cause of stereoscopic effect when a single 

 objective is used could be adduced than the mere fact and 



interests of those who may contemplate the acquisition 



'• of apparatus, it seems worth while to controvert the 



«. erroneously repeated in the last edition of Carpenter's Hand- 



the reflected image is an imperfect one. As made at present 



and lefl eye images are to all intents and purposes optically 



• :al. 



