On Orthoscopie and Psemloscopic Effects. By Prof. E. Ahhe. 209 



pencils uninvertecT, or whenever the Bamsden circle is an erect 

 image of the aperture ; and right-eye perspective is afforded 

 by the image from the left-hand half of the objective, -when- 

 ever those pencils at their emergence from the ocular are 

 inverted, or the Eamsden circle appears as an inverted image. 



This consideration has had nothing in view hitherto but the 

 different perspective characters of those partial images which may 

 be obtained in any monocular Microscope from opposite pdrtions of 

 the objective or from opposite halves of the Eamsden circle. Let 

 us now suppose any kind of hinocular arrangement, by which two 

 images of equal amplification and similar position are projected at 

 the same time, so as to be viewed with both eyes. All the foregoing 

 propositions must apply then to both these images at once without 

 any restriction or modification, because all conclusions hold good 

 for every arrangement of the optical system, whatever may be the 

 number of lenses, or prisms, or mirrors. In order to obtain true 

 stereoscopic impressions of an object in hinocidar vision, it will be 

 necessary and sufficient, therefore, that the right eye be affected by 

 the rays from the right-hand half, and the left eye by rays from the 

 left-hand half of that Eamsden circle which is depicted in front of 

 each eye, the opposite halves remaining ineffective ; pseudoscopic 

 vision will be obtained from the opposite arrangement, of course. 



Thus the proposition pointed out at the commencement of 

 this paper is shown to express the true and general condition of 

 either orthoscopie or pseudoscopic vision with the Microscope. 



In the ordinary binocular arrangements the rays which are 

 transmitted by opposite halves of the objective, are admitted to 

 different oculars separately. In this case, one half of the full 

 Eamsden circle only is depicted at the back of each eye-piece, 

 the other halves being stopped-off by interposed prisms. The 

 effect will be orthoscopie, therefore, whenever the two semicircles 

 appear in the position (Fig. 36), and it will be pseudoscopic 

 whenever they appear in the position P. Whether or not, under 

 these circumstances, orthoscopie action will require crossing-over 

 of the rays from the right-hand haK of the objective to the left eye- 

 piece, and vice versa, depends solely on the manner in which the 

 dehneating pencils are transmitted through the system, or the 

 apertm'e of the objective is projected. In a Microscope with non- 

 erecting ocular the emergent pencil is now inverted with reference 

 to the incident pencil. In the Wenham and the Nachet binoculars, 

 consequently, crossing-over is required, because the inversion of the 

 pencils is not changed by two reflections, and the Eamsden circle 

 represents still an inverted image. If the delineating pencils 

 have been reflected an even nuniber of times in the same plane, it 

 ■will be necessary for the rays to cross ; on the contrary, if they 



