538 THE STEREOSCOPE. 



picture. These lenticular prisms are generally set in 

 the top of a four-sided pyramidal box, -which is wider at 

 the bottom, where the pictures are placed, than at 

 the top. Fig. 294 shows a vertical section of the box; 

 p p are the lenticular prisms, s s is a partition within 

 the box which prevents each eye from seeing the 

 picture prepared for the other eye. Light falls into 

 the box through the opening o o upon the two pictures, 

 which are usually fastened side by side upon a piece of 

 cardboard ; this ' slide ' is introduced through the 

 horizontal opening e e. 



The aperture o o is generally closed by a moveable lid. If the 

 pictures are upon opaque paper the lid roust be opened, and the 

 stereoscope must be inclined so as to allow the light of the sky or 

 some other light to illuminate the pictures. If the pictures are on 

 transparent paper or glass the lid must be shut, and the instrument 

 held towards the sky or the artificial light, for which^ purpose the 

 bottom of the instrument is made of ground glass, or has two open- 

 ings, each of the size of the binocular picture. 



It is not always possible to form at once a single image of the 

 two pictures, but when we have succeeded in it, the appearance 

 of relief or solidity is most perfect and surprising. If we do not 

 succeed, the head should be moved a little so as to bring the eyes 

 nearer or farther from the instrument. Since the distance of 

 distinct vision and the distance between the eyes (about 7 cm ) 

 vary in different persons, the same stereoscope will not be suitable 

 for all eyes, unless some adjustment be attached which permits 

 an alteration in the relative distance between the prisms and the 

 pictures, and also in that between one prism and the other. The 

 absence of such an adjustment in most stereoscopes is the cause ot 

 one and the same instrument being well adapted for one person but 

 unsuitable for another. 



A convenient form of the instrument may be called ' stereoscopic 

 spectacles.' The two prisms are set into a kind of spectacle frame, 

 to which a handle is fixed, so that by moving them to or from the 

 pictures, distinct vision is obtained by adapting the distance to the 

 individual distance of vision of the person using them. In conse- 

 quence of the absence of any partition between the two pictures, 



