E. M. NELSON ON BINOCULARS. 



47 



unsaid, and if a careful reader will fill up the hiatus he will 

 then see that the professor's conclusions coincide with those of 

 Dr. Carpenter, excepting only that Abbe's are imperfect, and 

 do not meet every case. 



Prof. Abbe says that the eye-spots for orthostereoscopic 

 vision must be like this, Q D ; but he does not tell you that Q D 

 in an inverting microscope without a " cross-over," and in an 

 erecting microscope with a "cross-over," is an impossibility. 

 He says that the eye-spots arranged like this, DQ, give pseudo- 

 stereoscopic images ; but he does not say that in an inverting 

 microscope with a " cross-over," and in an erecting microscope 

 without a " cross-over," the attainment of that arrangement is 

 impossible. 



Prof. Abbe's law with regard to eye-spot pictures is only a 

 partial one ; suppose, instead of dividing the objective, you have 

 two whole objectives. How will his law help you ? The 

 Cherubin d' Orleans' binocular is a case in point. The eye-spots 

 are neither QD nor DQ, but are circular, and, according to 

 Abbe, the presence or absence of a " cross-over" has nothing to 

 do with the question. Therefore by the Abbe theory it is quite 

 impossible to state whether the Cherubin d' Orleans' binocular 

 is " ortho- " or "pseudo- " stereoscopic. According to Carpenter, 

 however, it is plain enough. The microscope 

 is inverting ; it has no " cross-over." The 

 image is, therefore, pseudostereoscopic. The 

 question also arises of the interpretation of 

 the images with a single objective, when a stop 

 like Fig. 4 is placed at the back of the objec- 

 tive, with, say, a Wenham 

 binocular. The eye-spot 

 images would be circular 

 (Fig. 14), and in the absen 

 of the unimportant (?) know- 

 ledge of either the trans- 

 position of the image or the "cross-over," how is one to say 

 whether the image is "ortho-" or "pseudo-" stereoscopic. 

 Here, again. Dr. Carpenter's theory meets the case, while that 

 of Prof. Abbe fails. 



The identity of Abbe's system, as far as it goes, with' that of 

 Carpenter can be clearly seen by tracing the rays through 



y 



