Royal Society. lul 



and shaded perspective drawings ; when, however, in the succeeding 

 ™ Talbot and Daguerre made their processes known, Mr. Wheat- 



of statues buildings, and even portraits of living person,, unicti, 

 wh!n presented in the stereoscope, no longer appeared as picture 

 but as P solid models of the objects from which they were taken. This 

 anDlication 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 

 e s dissimilar as that distance is greater, and, consequently as the 

 conve te"e of the optic axes becomes less. To a particular distance 

 TeWs a specific dissimilarity between the two pictures, and it is 

 a point of interest to determine what would take place on viewing a 

 nak of stereoscopic pictures with a different inclination of the optic 

 Keslhan that for which they were intended. The result of this 

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

 opKxes are neady parallel, the distances between th, : near -and 

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

 hand a P pair of pictures slightly dissimilar is seen w ^Jg 

 axes converge very much, the appearance is that of a bas-reiiet. 

 Is ro dfsagleable^r obviously incongruous effect is produced when 

 fwo pictures, intended for a nearer convergence of the optic axes 

 ^ a£n when the eyes are parallel or nearly so, we are able to aval 

 ourselves of the means of augmenting the perceived magnitude of 

 the b nocular image mentioned at the commencement of this abstract. 

 For this purpose the pictures, placed near the eyes, are caused to 

 col n de vvlien the optic axes are nearly parallel ; and the diverging 

 ras proceeding from the near pictures are rendered parallel by 

 lenses of short focal distance placed before the mirrors or prisms ol 



th Vome e0 adSnal observations were next brought forward respect 

 ine those" stereoscopic phenomena which the author, in his first 

 me § mo r ca led » conversions of relief." They may be produced in 

 ?\Z d Afferent ways :-lst, by transposing the pictures from one eye 

 the other; 2ndly, by reflecting each picture separately without 

 transposition; and 3rdly, by inverting the pictures to each eye se- 

 parately The converse figure differs from the normal figure in this 

 circumstance, that those points which appear most distant ,n the 

 latter, are the nearest in the former, and vice versd. 



An account is then given of the construction and effects of an 

 instrument for producing the conversion^ the re hef of any sohd 

 object to which it is directed. As this instrument conveys to the 

 S false perceptions of all external objects, the author calls it a 



• ',udoscope. It consists of two reflecting prisms, placed in a frame, 

 with adjustments, so that, when applied to the eyes, each eye may 



e rati- see the reflected image of the projection winch usually 



fal on that eye. This is not the case when the rcflex.on of an 

 ec's scen\n a mirror; for then, not only are the projections 



. , ,„, ( u1y reflected, but they are also transposed from one eye to 

 ' X? 3 Sierefore the conversion of re&* does nut take place. 



