150 Royal Suciety. 



it is viewed ; the pictures are also capable of sliding along these arms, 

 so that they may be simultaneously brought nearer to, or removed 

 further from, the mirrors. When the pictures remain at the same 

 distance and the arms are removed round their centre, the reflected 

 images, while their distances from the eyes remain unchanged, are 

 displaced, so that a different inclination of the optic axes is required 

 to cause them to coincide. When the arms remain in the same 

 positions and the pictures are brought simultaneously nearer the 

 mirrors, the reflected images are not displaced, and they always co- 

 incide with the same convergence of the optic axes ; but the mag- 

 nitude of the pictures on the retinae becomes greater as the pictures 

 approach. The experimental results afforded by this apparatus, so 

 far as regards the perception of magnitude, are the following : the 

 pictures being placed at such distances, and the arms moved to such 

 positions, that the binocular image appears of its natural magnitude 

 and its proper distance, on the arms being moved so as to occasion 

 the optic axes to converge less, the image appears larger, and on 

 their being moved so as to cause the optic axes to converge more, 

 the image appears less ; thus, while the magnitude of the pictures 

 on the retina? remains constantly the same, the perceived magnitude 

 of the object varies, through a very considerable range, with every 

 degree of the convergence of the optic axes. The pictures and arms 

 1 eiug again placed so that the magnitude and distance of the object 

 appear the same as usual, and the arms being fixed so that the con- 

 vergence of the optic axes does not change ; while the pictures are 

 brought nearer the mirrors the perceived magnitude of the object in- 

 creases, and it decreases when they are removed further off; thus, 

 w-hile the inclination of the optic axes remains constant, the per- 

 ceived magnitude of the object varies with every change in the mag- 

 nitude of the pictures on the retina;. After this the author takes 

 into consideration the disturbances produced in our perception of 

 distance under the same circumstances, and concludes that the facts 

 thus experimentally ascertained regarding the perceptions of magni- 

 tude and distance, render necessary some modification in the pre- 

 valent theory regarding them. 



The author next reverts to the stereoscope and its effects. He 

 recommends the original reflecting stereoscope as the most efficient 

 instrument, not only for investigating the phenomena of binocular 

 vision, but also for exhibiting the greatest variety of stereoscopic 

 effects, as it admits of every required adjustment, and pictures of 

 any size may be placed in it. A very portable form of this instru- 

 ment is then described, and also a refracting stereoscope suited for 

 Daguerreotypes and small pictures not much exceeding the width 

 between the eyes. In the latter instrument the pictures are placed 

 side by side and viewed through two refracting prisms of small 

 angle which displace the pictures laterally, that on the right side 

 towards the left, and that on the left side towards the right, so that 

 they appear to occupy the same place. When the first part of these 

 investigations was published the photographic art was unknown, 

 and the illustrations of the stereoscope were confined to outline 



