190 Scientific Proceedings, Royal Dublin Society. 
omit this last re-transfer to make the pictures right for. this 
stereoscope. 
The last form of stereoscope I have to bring before your notice 
is one peculiarly suited for transparent photographs. It will be 
easily understood from Fig. 4, Plate 11. By a combination of 
prisms and lenses images of the right and left pictures of an 
ordinary stereoscopic slide (S) are formed superposed on the 
surface of a concave mirror (C). When the observer stands 
opposite the mirror at a certain distance, the pencils of light 
which form the right picture are reflected by the mirror into his 
left. eye, and those which form the left picture are reflected into 
his right eye. The result is apparently an acrial image of the 
object in beautiful relief. 
For transparencies of almost all subjects, but especially statuary, 
this form of stereoscope is most admirably suited ; and the fact 
of not having to look through two apertures—combined with that 
of the pictures as seen with both eyes being actually superposed 
and not side by side, and, therefore, that there can, by no possi- 
bility, be any difficulty of causing the images to coalesce— 
renders this form of instrument a great favourite with most ob- 
servers. 
