204 SEC. 7. LIGHT. 



804. Telescope, No. 14. 



Voigtldnder and Son ( Chevalier von Voigtlander), Bruns- 

 wick. 



807. Iceland Spar Ball, for showing double axis. 



A. Hilgcr. 



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. 



810a. Globe made of Spar. M. Lutz, Paris. 



811. Early form of Stereoscope. 



The Council of Kings College, London* 



812. Early form of Stereoscope. 



The Council of King's College, London. 



812a. Polistereoscope. Apparatus which serves as tele- 

 stereoscope, pseudoscope, iconoscope, &c., &c. 



Augustus Rigid, 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 ; 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 sameobject 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 afterwards 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 telestereoscopc. 



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, together with the 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 for the large mirror. 



Fig. 4. In Fig. 4 the apparatus is placed so that the ej-es see the objects as 

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

 eyes ; the relief in objects made with vertical wires then disappears. If the 

 diaphragm which bears the two tubes is kept fixed, and the instrument turned 

 slowly round the left tube, very curious apparent motions occur in the objects 

 under observation. 



For the mathematical theory of this apparatus, see the " Nuovo Cimento," 

 2 d ser. t. xiv. 



