274 Transactions of the [^"Sillll^lSeTi'm 



effect in all optical instruments by making their glasses as thin as 

 possible. 



Hence total-reflexion prisms, by their unavoidable thickuess and 

 the number of their surfaces, produce indistinctness that cannot be 

 compensated or removed, and give to the pictures of an instrument 

 in which they are used an unequal appearance and value, the blend- 

 ing of which, by two eyes, can never produce a correct stereoscopic 

 relief. 



Largeness of aperture in the object-glass gives a certain reHef to 

 an object under view with one eye ; this is simply monocular rehef, 

 and is untruthfully exaggerated by the two oblique reflexions in 

 the prism, giving a lateral elongation to its image : thus the view 

 of the left eye being rendered dissimilar, presents an appearance of 

 rehef when blended with the undistorted view of the right eye. 



The amount of this distortion may be seen in the diagram of 

 the prism, &c., in which the two paths of the light to the right and 

 left eye are p-oduced hackwards until they intersect the plane of 

 the object. 



One path is vertical, the other oblique. Now the area of the 

 oblique section is greater than that of the rectangular section, and 

 if the area containing the object is distorted, the object is distorted 

 also. Again, as the eye refers the position of an object to the 

 direction from whence the hght came, we may reasonably infer that 

 the left eye sees an object considerably to the right of the real one, 

 and the ohliquity and lateral displacement have the effect of making 

 a Cube appear as a Parallopipedon, and a Sphere as an Ellipsoid, 

 and these forms superposed by any stereoscope will give a false per- 

 spective of the Cube and Sphere. 



From the above considerations, believing that the attempt to 

 divert half the hght from an objective to form a secondary image 

 by a total-reflexion prism, to be incapable of improvement, I have 

 experimented in other directions, and have even gained more light 

 and more equality by the use of two prisms, having only a single 

 reflexion in each. 



In January, 1869, I contributed a paper to the Quekett Club, 

 on some experiments made in the year 1858, which was read by 

 my co-member and friend Mr. George, and to which I now advert 

 only as a matter of history. 



In the microscope therein described, I proposed to divide the 

 light from the objective into three portions and direct one half into 

 each eye, through the medium of two reflectors and eye-pieces. The 

 object-glass shown to the meeting was a hemispherical plano-convex 

 lens, and the plane side had two reflecting facets at the binocular 

 angle. This lens was to be placed over the object with its angu- 

 lated surface at an angle of 45° to the perpendicular, in which 

 direction the rays from the object should reach it from below, to be 



