642 THE SENSES. 



imitated. Such is the. principle of the instrument known as the stereo- 

 scope. This is simply a box or framework, holding two photographic 

 pictures of the same object, which have been taken from two different 

 points of view, corresponding to the different positions of the two eyes. 

 Thus one of the pictures represents the object as it would in reality be 

 seen by the right eye, and the other represents it as it would be seen 

 by the left. When these pictures are so placed in the stereoscope that 

 each eye has presented to it the appropriate view, the two images, occu- 

 pying the point of fixation, are fused upon the retina, and produce an 

 extremely deceptive resemblance to the projection and stolidity of the 

 real object. 



The acuteness of perception, by which the eyes appreciate a slight 

 difference in the two retinal images, is the measure of what may be 

 called their stereoscopic sensibility. It has been observed that two 

 coins, composed of different metals, but struck from the same die, are 

 slightly different in volume, owing to the unequal dilatation of the 

 metals after receiving the impression of the die. This difference may be 

 quite inappreciable to the eye in ordinary examination, even when the 

 coins are placed in contact with each other ; but if they be made to take 

 the place of the two pictures in a stereoscope box and viewed together, 

 the resulting image, instead of presenting a plane surface, appears ob- 

 lique and convex. 



The degree of stereoscopic sensibility was tested by Helmholtz in the 

 following manner: Three metallic pins were fixed upright in small 

 movable blocks of wood, placed side by side, so that the pins sh6uld be 

 about 12 millimetres distant from each other, and nearly in the same 

 vertical plane. The observer then, using both eyes simultaneously, 

 examined the appearance of the objects from a distance of 340 milli- 

 metres, the pins being arranged at right angles across the line of view. 

 The immediate object of the examination was to determine, from the 

 stereoscopic effect, whether the three pins were placed exactly in the 

 same plane, or whether either of them were in advance of or behind the 

 others. It was found possible to detect in this way a deviation in posi- 

 tion of one of the pins equal to one-half its own thickness, that is, 

 0.25 mm. ; and the deviation was recognized with absolute certainty 

 when it amounted to the entire thickness of the pin, that is, 0.50 milli- 

 metre. 



General Laws of Visual Perception. Beside the laws regulating the 

 formation and combination of optical images, there are certain pheno- 

 mena connected with visual perceptions in general, which are of con- 

 siderable importance in the physiology of sight. Some of these phe- 

 nomena require for their study special modes of investigation, while 

 others are made evident by comparatively simple means, and are often 

 of consequence in their hygienic relations. 



Luminous impressions upon the eye remain for a certain time 

 after the cessation of the light. The persistence of luminous impres- 

 sions thus left upon the eye is very short, and is not usually noticeable, 



