139 



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

 on the retinae 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 

 being 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 the}^ are removed further o& ; thus, 

 while 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 retinse. After this the author taljes 

 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 eliects. 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 ever}' 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 

 Daguerreotvpes and small pictures not much exceeding the \^ddth 

 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 Vv^ere confined to outline 

 and shaded perspective drav/ings ; when, however, in the succeeding 

 year, Talbot and. Daguerre made their processes knovrn, Mr. Wheat •- 

 stone v/as enabled to obtain binocular Talbotypes and Daguerreotypes 

 of statues, buildings, and even portraits of living persons, which, 

 when presented in the stereoscope, no longer appeared as pictures, 

 but as solid models of the objects n-om which they were taken. This 

 application was first announced in 1841. 



The two projections of an object, seen by the two eyes, are dif- 

 ferent according to the distance at which it is viewed ; they become 

 less dissimilar as that distance is greater, and, consequent!}^, as the 

 convergence of the optic axes becomes less. To a particular distance 

 belongs a specific dissimilarity betv\'een the two pictures, and it is 

 a point of interest to determine vvhat v^-ould take place on viewing a 

 pair of stereoscopic pictures with a different inclination of the optic 

 axes than that for which they were intended. The result of this 

 inquiry is, that if a pair of very dissimilar pictures is seen when the 

 optic axes are nearly parallel, the distances between the near and 

 remote points of the object appear exaggerated ; and if, on the other 



