THE SENSE OF VISION. 803 



obtained with the outlines of geometrical solids, photographs of coins or medals 

 or of objects which may readily exist in an inverted form. Where the photo- 

 graphs represent objects which cannot be thus inverted, such as buildings and 

 landscapes, the pseudoscopic effect is not readily produced another example 

 of the power (see p. 800) of the outline of a familiar object to outweigh other 

 sorts of testimony. 



A pseudoscopic effect may be readily obtained without the use of a stereo- 

 scope by simply converging the visual axes so that the right eye looks at the 

 left and the left eye at the right picture of a stereoscopic slide. The eyes may 

 be aided in assuming the right degree of convergence by looking at a small 

 object like the head of a pin held between the eyes and the slide in the manner 

 described on p. 758. Figure 260, viewed in this way, will present the appear- 

 ance of a hollow truncated cone with the base turned toward the observer. A 

 stereoscopic slide with its pictures reversed will, of course, when viewed in this 

 way, present not a pseudoscopic, but a true stereoscopic, appearance, as shown 

 by Figures 226 and 227. 



Binocular Combination of Colors. The effect of binocularly combin- 

 ing two different colors varies with the difference in wave-length of the colors. 

 Colors lying near each other in the spectrum will generally blend together 

 and produce the sensation of a mixed color, such as would result from the 

 union of colors by means of the revolving disk or by the method of reflected 

 and transmitted light, as above described. Thus a red and a yellow disk 

 placed in a stereoscope may be generally combined to produce the sensation 

 of orange. If, however, the colors are complementary to each other, as blue 

 and yellow, no such mixing occurs, but the field of vision seems to be occupied 

 alternately by a blue and by a yellow color. This so-called " rivalry of the 

 fields of vision " seems to depend, to a certain extent, upon the fact that in 

 order to see the different colors with equal distinctness the eyes must be dif- 

 ferently accommodated, for it is found that if the colors are placed at different 

 distances from the eyes (the colors with the less refrangible rays being at the 

 greater distance), the rivalry tends to disappear and the mixed color is more 

 easily produced. 



An interesting effect of the stereoscopic combination of a black and a 

 white object is the production of the appearance of a metallic lustre or polish. 

 If, for instance, the two pictures of a stereoscopic slide represent the slightly 

 different outlines of a geometrical solid, one in black upon white ground and 

 the other in white upon black ground, their combination in the stereoscope 

 will produce the effect of a solid body having a smooth lustrous surface. 

 The explanation of this effect is to be found in the fact that a polished surface 

 reflects the light differently to the two eyes, a given point appearing bril- 

 liantly illuminated to one eye and dark to the other. Hence the stereoscopic 

 combination of black and white is interpreted as indicating a polished surface, 

 since it is by means of a polished surface that this effect is usually produced. 



Corresponding Points. When the visual axes of both eyes are directed 

 to the same object two distinct images of that object are formed upon widely 



