14 Transactions of the Society. 



of depth lies in the fact that they are observing with both eyes. 

 The certain expectation that "Now I shall see objects stereoscopic- 

 ally," suffices to induce the apparent effect. This suggestion, with 

 i lie impression of greater viridity, produces, by the binocular 

 Microscope, the impression of stereoscopic effect and life-like 

 appearance. 



8. l'AKALLACTIC EFFECTS. 



Although parallactic perception of depth is out of the question 

 with the new instrument, and the effect which sometimes astounds 

 the observe)' is only psychological with higher magnifications, 

 nevertheless conditions may be obtained with the binocular Micro- 

 scope resulting in proper parallactic vision, and what is more both 

 orthoscopic and pseudoscopic. This takes place when the eyes 

 of the observer are not centred with the eye-pieces. One has 

 only to take care that the half of the rays emanating from the 

 object should reach each eye, and, moreover, on account of inver- 

 sion in the Microscope, the rays from the left half of the object 

 must be led to the right eye, and from the right half to the left 

 eye if you desire an orthoscopic effect. In the reverse case we 

 have pseudoscopic effect, i.e. parts which stand in relief appear to 

 be depressed, and so on. These conditions were first made clear 

 by Professor Abbe in 1882.* 



As is shown in fig. 3,f this screening has to be done in the 

 upper focal plane of the objective. It could, however, be moved 

 to an image of this focal plane — the only one available in the 

 ordinary Microscope being the Eamsden disk. There one would 

 have to apply to the " exit pupil " as has been done by Abbe, 



* On the Conditions of Orthoscopic and Pseudoscopic effects in the Binocular 

 Microscope. Journ.R.M.S., 1881, pp. 203-11. 



t Figure 3 shows the path of the rays in the Microscope from the image 

 of an object P Q to an eye which looks into the Microscope from a position 

 out of the direct line of the principal axis. The rays from P are indicated by 

 dotted lines, those from Q by continuous lines. Both points are represented on 

 the retina, hence the field of vision will not be limited. Of the eight rays 

 proceeding from the object, pairs may be taken together which are parallel in 

 front of the objective and therefore intersect in the upper focal plane of the 

 objective. This focal plane is represented by means of the eye-piece in the 

 Eamsden disk of the whole Microscope. If the eye of the observer is out of the 

 line of the principal axis, some of the rays are prevented by the iris from reaching 

 the inner eye. This is the case with all those rays in the figure which pass 

 through one half of the focal plane of the objective, that is to say, only such rays 

 contribute to form the image of the object PQ as run in a certain direction. 

 In the example given in the figure only the shaded part of the path of the rays 

 reaches the eye. If the other eye of the observer is so placed that it receives the 

 other half of the path of the rays, then the two eyes receive two images of 

 different perspective, and all the conditions of a stereoscopic perception of depth 

 are fulfilled. If on this assumption the eye in the figure is a right eye, the 

 observer will receive a pseudoscopic image ; if, on the other hand, it is a left eye, 

 the observer will receive an orthoscopic one. (We suppose ourselves to be 

 opposite the observer.) 



