Mr. Charles Hussey, on Stereoscopie Photograplis. 149 



ossicles of a crinoid ; another, a fragment of coral ; two more 

 contain organisms that may be Forams, which have been 

 encrusted or altered ; and the three which remain may be parts 

 of the spines of an Echinoderm. 



67. — Stereoscopic Photographs, and how to take them. 



By Charles Hussey. 



(Read at a Meeting of the Photographic Section, April 6th, 1888.) 



Twenty-five years ago a stereoscope was found in every 

 drawing-room ; now such an instrument is hardly ever seen. 

 The very success of the instrument caused its downfall, as the 

 large demand for stereoscopic photographs induced the production 

 of views which were either not stereoscopic, being prints from 

 the same negative mounted side by side, or else prints which, 

 being stereoscopic, were wrongly or imperfectly mounted. With 

 the faulty views were found badly-constructed instruments, and 

 the public soon lost confidence in the stereoscopic system. 



No one who has ever perfectly realised stereoscopic effect will 

 forget his first delight at the marvellous sensation of relief 

 revealed to him ; and now that stereoscopic photography is 

 showing signs of revival, it may interest beginners to know how 

 stereoscopic photographs are produced. The present paper is 

 written in the hope that some members of the Croydon Club may 

 be tempted to take up this fascinating branch of our art-science. 



Briefiy stated, stereoscopic effect is due to parallax ; that is, 

 the same object is seen by the two eyes from different points of 

 view, the "sensation of relief" or appearance of solidity being 

 produced by the varying convergences of the axes of the eyes 

 in looking at an object. 



Suppose the line a, b, to represent the distance of the eyes 

 apart, or, in other words, the parallactic base of the triangle, 

 the eyes themselves being directed to the points o, d, e, at 



b2 



