LXXIV.] 



STEREOSCOPE. 



361 



FIG. 282. To illustrate Haploscopic Vision. 



parallel tubes placed one in front of each eye, one obtains two 

 different retinal pictures. Nevertheless, single vision is the result, 

 and the two different pictures are combined to give an illusory 

 sensation of one object. One gets approximately haploscopic vision 

 with a stereoscope. 



Haploscopic vision may be illustrated by vertical lines, parts of 

 circles (Hering, Hermann's Handbucli d. Physiologie, iii. p. 355), 

 or by the familiar bird 

 and cage experiment (fig. 

 282). Hold the figure 

 close to the eyes, separate 

 the two fields of vision 

 by a card held vertically 

 in the mesial plane be- 

 tween the eyes, and look 

 beyond the picture, i.e., 

 allow the eyeballs gradu- 

 ally to diverge from the point of convergence. On doing so, as 

 the visual axes become less convergent, one has on the right visual 

 field a bird, on the left a cage, the bird appears to move into the 

 cage, and in consciousness we have the illusion as if the bird were 

 in the cage. 



15. Stereoscope. 



(a.) Examine a series of stereoscopic slides to show the combina- 

 tion of the images obtained by the right and left eyes respectively. 



(b.) Struggle of the Fields of Vision. Place in a stereoscope 

 a slide of glass with vertical lines ruled on one half of it and hori- 

 zontal lines on the other half. Look at the two dissimilar images ; 

 note that they are not combined, but sometimes one sees it may be 

 only the horizontal, at another only the vertical lines. It may be 

 done also with coloured slides. 



(c.) Lustre. Use a stereoscopic slide, preferably a geometrical 

 pattern, e.g., a crystal where the boundary-lines are white and the 

 surfaces black. Such a slide shows glance or lustre. 



3 6. Lustre in Coloured Objects. 



This may be shown by looking at a green patch (electric green) on a red 

 ground through coloured glass, e.g., a blue glass before one eye and a red one 

 before the other eye. Other combinations may be made. 



17. Stereoscopy Dependent on Differences of Colour. 



(a.) Difference of colour may be the cause of an apparent difference in 

 distance. If one looks from a distance of 3 metres at red and blue letters 

 (8x4 cm.) on a black background, to most observers the red appears nearer 

 than the blue. It is usual to explain this by difference of accommodation, 

 more effort being required to focus for the red letters than for the blue ; and 



