I. DISTKIBUTOKS. 175 



807. An Iceland Spar Ball. A. Hilger. 



808. Lens, by Stacker, modified by Dr. Snellen. The 

 system indicated above lias been utilized with two cylindrical 

 glasses ( x 12= 12). When the cylindrical surfaces are parallel, 

 their refraction becomes annulled ; when they are crossed, they 

 become added. The axis is fixed for the same reasons as above. 



M. Crete s, Paris. 



810. Series of Metrical Glasses. The dioptric unit 

 is a lens of one metre focus ; the lens 0*50 to two metres 

 focus is a semi-dioptric value of the unit ; the lens 2 to 50 

 focus is a dioptric value of double the unit. The same rule 

 applies to all the other lenses in the collection. M. Cretes, Paris. 



81Oa. Globe made of Spar. M. Lutz, Paris. 



811. Early form of Stereoscope. 



Wheatstone Collection of Physical Apparatus, Kings 

 College, London. 



812. Early form of Stereoscope. 



Wheatstone Collection of Physical Apparatus, King's 

 College, London. 



S12a. The Polistereoscope. Apparatus which serves as 

 telestereoscope, pseudoscope, iconoscope, &c., &c. 



Augustus Righi, Professor of Natural Philosophy, Royal 

 Technical Institute, Bologna (Italy). 



This apparatus consists of two plane mirrors, one of which (on the left in 

 the figures) can turn about an horizontal and a vertical axis j the other 

 mirror, besides these movements, can be fixed at different distances from the 

 former. The eyes must be applied at two cylindrical tubes fixed to a dia- 

 phragm, which can take different positions. One of the eyes sees directly the 

 objects/while the other sees the same object but apparently in a different position. 

 This virtual position can be determined by forming the image of the eye, given 

 by the left mirror, and after the image of the point so determined in relation 

 to the other mirror. If objects not too near are observed the illusion succeeds 

 equally, though the image in the eye which sees by reflection is smaller than 

 the other. According to the inclination which is given to the mirrors, it is 

 possible to make any determinate point of the observed objects appear in the 

 true position. 



Fig. 1. If the apparatus be placed as in Figure 1, it produces the effect of 

 the telestereoscope. 



Fig. 2. Placed as in Figure 2, it acts as a pseudoscope. According to the 

 distance between the mirror, diminution or augmentation of relief can be 

 obtained, or an inversion of relief. Some curious effects (which cannot be 

 obtained with a Wheatstone's pseudoscope) are observed by looking at 

 rotating geometrical solids, constructed with metallic wire, or by looking at 

 these solids while the observer moves round them. 



Fig. 3. With the apparatus placed as in Figure 3, the effects of an icono- 

 scope are obtained. A very narrow mirror is substituted. 



Fig. 4. In Fig. 4 the apparatus is placed so that the eyes see the objects as 

 if they were in a same plane perpendicular to the right line which joins the 



