210 Transactions of (lie Society. 



have been reflected an odd number of times, it is not only un- 

 necessary, but is destructive of ortboscopic effect, provided ordinary 

 (non-erecting) eye-pieces are applied. Hence in Stephenson's 

 binocular such crossing-over is not required (and would give 

 pseudoscopic effect) because the reflecting prism makes the final 

 emerging pencils by one reflection conformable to the incident 

 pencils, and projects the aperture of the objective as an erect 

 image (the plane of the two axes being considered only). 



According to general dioptrical propositions, uninverted trans- 

 mission of the pencils from the object to the image (and erect 

 position of the Eamsden circle) is always allied to an erect image, 

 and vice versa. The position of the microscopical image (whether 

 erect or inverted) may therefore be used in order to determine 

 the conditions of ortboscopic or pseudoscopic vision. It ought to 

 be borne in mind, however, that this criterion does not indicate 

 the causa efficiens of the one or the other kind of action, but rather 

 an accessory indication only. 



As to those other binocular arrangements by which two images 

 are produced without a geometrical division of the rays from 

 the opposite halves of the objective, each eye-piece projects a full 

 Eamsden circle, and thus simultaneously embraces an image with 

 both right and left eye perspective. Of course these are non- 

 stereoscopic in themselves, as is the case, for instance, with Powell 

 and Lealand's arrangement, and with that device which was pointed 

 out by Mr. Wenham in 1866 and applied in the author's stereo- 

 scopic ocular. With binoculars on such a principle either kind 

 of stereoscopic action may be obtained at pleasure by suppress- 

 ing one-half of each Eamsden circle. Suppressing, by suitable 

 diaphragms at the exact plane of the Eamsden circle,t the inside 

 halves (the right half for the left eye and the left half for the right 

 eye) of both circles, ortboscopic eflfect is produced ; pseudoscopic, 

 when the outside halves are stopped ofil 



Moreover, the difference of perspective between an image pro- 

 jected through the centre of the aperture (or of the Kamsden cu'cle) 

 and another image projected through an excentrical portion, will be 

 half that difference which is obtained from opposite excentrical 

 portions, but vsdll retain the same character still. For obtaining 

 stereoscopic action with binocular arrangements of the kind here 

 referred to, it will be sufficient, therefore, that one Eamsden circle 

 only be half-stopped, the other being left active in its full extension. 

 Ortboscopic efiect (though with less difi'erence of perspective) will 



t This condition is necessary for obtaining the bisection of the emergent 

 pencils from all points of the field at the same time. Diaphragms at any other 

 plane (whore the emergent pencils have not a common cross-section) would, of 

 course, encroach more or less upon the field of vision. If, however, the above 

 condition is fulfilled, there cannot be any encroachment on the field. 



