64 E. M. NELSON ON lUNOCULAllS. 



elements (lenses, prisms, mirrors, etc.) may be components of 

 the optical arrangement." 



A few pages farther on we read : — * " Whether or not under 

 these circumstances orthoscopic action will require crossing- 

 over of the rays from the right hand half of the objective to 

 the left eye piece and vice versa depends solely on the manner 

 in which the delineating pencils are transmitted through the 

 system." On the same page he says : — " In the Wenham and 

 the Nachet binoculars consequently crossing over is required; " 

 and on the next page : "In Stephenson's binocular such cross- 

 ing over is not required." Thus the things Avliich are " im- 

 material " on page 204 become under precisely the same condi 

 tions " essential " on page 209. 



Abbe's conditions are — 1st, non-essential ; 2nd, incomplete ; 

 3rd, misleading ; 4th, in no instance do they conti*avene Car- 

 penter's dictum ; and 5th, with regard to the action of the 

 pupils of the eyes they are incorrect. 



We have now come to the second portion of the subject, viz., 

 the origin of the dissimilar images. 



Carpenter says the origin is " perspective," but Abbe "paral- 

 lactic displacement." 



Stereoscopism is a difficult subject, and one which for sol ation 

 does not lend itself entirely to mathematical demonstration. It 

 is so inseparably mixed up with mental action that it can hardly 

 be dealt with by either optical or mechanical lines of argument. 



Stereoscopism or " solid view " can be obtained by one eye, 

 for if you shut one eye a book appears solid with the other eye, 

 but solidity is better and more perfectly seen with two eyes. 



With simple microscopes (loupes) solidity is manifest, 

 although only one eye is used. But what in these cases makes 

 an enormous difference is the way the object is looked at. Thus 

 semi-transparent objects, with transmitted light, exhibit very 

 little solidity, while the same objects viewed by I'eflected light 

 appear moi-e solid with one eye than when seen in an ortho- 

 stereoscopic binocular by transmitted light with two eyes. 

 The light and shade which is secured by the employment of 

 reflected light and lost with transmitted light is the cause of 

 this heightened effect. 



A curious instance of stereoscopic effect with a single picture 



* "R. M. S. Journal,'* 1881, p. 209. 



