Mr. W. Hardie^s Description of a New Pseudoscope. 445 



new pseudoscope increases the difference of the retinal images is 

 illustrated by fig. 3, in which also an attempt is made to indi- 

 cate the nature and mode of production of the binocular effects. 

 In this drawing R and L are intended to represent the right and 

 left eyes ; O and o two real objects at different distances ; Ir and 

 ir the reflected images of these objects seen by the right eye 

 alone ; 11 and il the images seen by the left eye alone ; and B 

 and b the binocular images of the two objects, resulting from 

 the combinations of the reflected images when viewed by both 

 eyes at once. All these reflected and binocular images are repre- 

 sented as near as may be in their true places, positions, and 

 forms relative to the objects O and o, and the rest of the draw- 

 ing. The thin dotted lines are introduced to indicate more 

 clearly the directions of the various visual rays after their last 

 reflexion, so that the connexion of each reflected image with its 

 proper object may be more distinctly traced. Now in order to 

 understand this illustration, the following facts must be borne in 

 mind, namely, that the relative position as regards right and 

 left of any two visual rays entering one eye corresponds with the 

 observed relative positions of the objects from which they pro- 

 ceed ; and also that the angle which these rays make with one 

 another at their intersection in the centre of the eye agrees with, 

 and may be adopted as a certain index of the real distance from 

 one another of the images of their objects in the picture upon 

 the retina. Applying these facts, then, to the diagram, it will 

 there be seen, that, supposing the instrument removed, the right 

 eye would see the more distant object o situated to the right of 

 the nearer one 0, while the left eye sees it on the left of ; the 

 angles ORo and OLo being the measures of the distances which 

 separate the images of these two objects in each retinal picture. 

 This shows that the natural difference between the two retinal 

 pictures is, that the images of the two objects in the one occupy 

 a relative position the reverse of what they occupy in the other ; 

 and that the sum of the angles ORo and OLo consequently 

 represents the amount of that difference. In the same way it 

 may be shown, with respect to the reflected images seen by either 

 eye alone, when the instrument is interposed, that the sum of 

 the angles Ir-R-2> and ll-h-il must represent the amount of the 

 difference between them ] but these angles respectively include 

 the other two ; their sum must therefore be greater, and conse- 

 quently so must the difference it represents be greater than that 

 represented by them. 



I have now only to mention that either form of this pseudo- 

 scope makes an excellent stereoscope. With drawings of con- 

 vertible forms, the first described instrument (fig. 1) is capable 



