If) Transactions of the Society. 



sary screening can, however, be done more conveniently in another 

 way, by placing the pupil of the eye in a special way in the path 

 of the rays. For example, if we make the space between the 

 oculars somewhat less than the distance between the observer's eyes 

 would be for parallel vision ; but if, nevertheless, his eyes remain 

 parallel and wholly without strain, then he must necessarily per- 

 ceive an orthoscopic effect; on the other hand, pseudoscopic effects 

 must be expected if the oculars are farther apart than the mean 

 distance between the observer's eyes. 



This consideration, which is a direct result of Abbe's theories, 

 was proposed by A. C. Mercer.* Observation confirms its truth 

 entirely for low power magnifications. With higher powers the 

 Eamsden disk becomes so small that, probably owing to the move- 

 ments of the eye, it cannot be observed in part but can only be 

 taken in entirely or not at all. 



This phenomenon can, however, best be observed with incident 

 light, as the production of cast shadows is most conducive to an 

 increase of stereoscopic effect. Besides all granular preparations, 

 for instance, somewhat thicker test preparations of Macroglossa 

 stdlatarum are most suitable for observing this effect. The oblique 

 illumination is best obtained in this case by fitting a concave 

 Microscope mirror with spindle into one of the holes for the speci- 

 men clamps ; the light may then be directed obliquely on to the 

 preparation, the individual scales throwing shadows on each other 

 and even at times on themselves. Coins are also very suitable 

 with low-power magnifications. For this case with a suitable 

 adjustment of the distance between the eye-pieces, one sees the 

 lettering stand out with remarkable parallactic effect. 



In conclusion, it may again be remarked that with medium and 

 still more high power magnifications there can be no question of 

 an actual parallactic effect. The advantage of the binocular 

 Microscope lies in such cases in the qualitatively enhanced im- 

 pression produced in different ways, and above all in its hygienic 

 importance. 



* Stereoscopic Vision with non-stereoscopic Binocular arrangements. Journ. 

 R.M.S., 1882, p. 271. 



