226 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



possible to discuss the conditions of the allied subject — viz. the change 

 whereby the perspective arising in the space-object is presented to the 

 eye. That change is closely connected with the above condition. It 

 must at the same time be supposed that the observer's eyes remain 

 accommodated for infinity. Yet it cannot be asserted that these 

 conditions as to accommodation-adaptability of the eyes are ever com- 

 pletely satisfied. But it will be difficult to deny the authors this 

 simplifying assumption if the similar condition be conceded without 

 hesitation in the clearer case of monocular instruments. In any case 

 the conditions should be plainly and clearly laid down on which the 

 consequent explanation depends. 



In many instruments intended for binocular use there is no space- 

 image, in the sense of geometrical optics, owing to the fact that in 

 many cases the rays do not intersect, but merely cross one another. 

 But "since the observer, even in such instances, not infrequently receives a 

 uniform impression, the explanations must be sought for in physiological 

 rather than in geometrical optics. The result of the authors' view, 

 therefore, is to very much narrow down the ground on which binocular 

 instruments should be treated, and to lay a sure foundation for 

 explaining the construction of selected forms. The space-image, in the 

 strict sense, will only arise when the axes-directions of the system 

 serving both eyes are parallel in both the space-object and in the space- 

 image, and when there is also exact and similar correspondence between 

 the planes of the object and the planes of the images. 



Moveover, it is possible to consider systems with a common objective 

 — under these circumstances the space -object possesses only a single 

 position-plane — or with both systems completely separated, and set up 

 parallel to each other. 



The authors then discuss the application of their princples to double 



loups. 



(2) Eye-pieces and Objectives. 



Watsons' 1/6 and 1/12 Objectives.* — The essential features claimed 

 for these lenses (figs. 20, 21) are the capability of bearing high eye-pieces 

 without breaking down ; the capability of utilising a large solid cone of 



Fig. 20. 



Fig. 21. 



illumination ; perfect centring ; a definition which leaves no doubt as 

 to the structure examined. 



* Watson and Sons' Special Catalogue. 



