150 Mr. Charles Hussey on 



different distances, it is obvious that the convergence of the optic 

 axes must be greater in the case of a, c, b than a, d, b, and so on, 

 and it is this greater or less convergence which gives the sen- 

 sation of distance. Nature has provided man with muscles to 

 alter the curvature of the lens of the eye ; and by this means, 

 and by habit and use, we may fancy we can see relief with one 

 eye only ; but it is not so, as the area of the pupil of the eye, 

 even if distended to its utmost, as by the application of bella- 

 donna, is altogether inadequate to produce parallax. 



Again, if we are looking at very distant objects, such as moun- 

 tains, the distance of the eyes apart, that is, the parallactic base 

 of the triangle, is so small, and the angle at the apex of the 

 triangle so acute, that the sensation of relief is lost ; it is there- 

 fore necessary for some subjects to take the two pictures at a 

 considerable distance apart, so that the base of the triangle may 

 be lengthened. Some successful stereos of the moon have been 

 obtained by photographing one picture in England and the other 

 in the Antipodes. This, of course, is an extreme case ; but the 

 same result is observable in, say, a rocky coast-line taken at a 

 considerable distance out at sea. If one picture be exposed, and 

 the vessel allowed to travel some yards before the other ia 

 uncapped, a sensation of relief will be obtained in the resulting 

 prints, which is not visible in nature. In this case it is needless 

 to point out that foreground is sacrificed. 



That the two eyes do see differently may be easily proved. 

 If one eye be closed, and any near object, say, a window-frame in 

 the room, be covered with a finger at arm's length, and then the 

 other eye be opened and the first one closed, it will be seen at 

 once that the finger occupies quite a different position in the field 

 of vision of the second eye ; this effect is due to the convergence 

 of the optical axes to a near point, or, in other words, to parallax. 

 Now in order to apply this principle to photography it is only 

 necessary to take pictures from two different standpoints, corre- 

 sponding more or less approximately to the relative positions of 

 the eyes, and then by means of a stereoscope to cause these two 

 pictures to blend in the brain to produce stereoscopic effect. 



Special cameras are provided for this work, fitted with twin 

 lenses, that is, two lenses of identical foci, but any camera and 

 lens which will take a photograph may be employed with success, 

 the possessor of the humble quarter-plate being as well equipped 

 as the owner of the lordly twelve-by-ten. 



For sizes below half-plate it is necessary to take one photo- 

 graph, and then, by moving the camera bodily a few inches, to 

 expose another plate for the same time, taking care that the 

 object in both cases occupies the same position on the plate. 

 This may be done by moving the camera with its stand ; but a 

 better way is to fit the camera on a board with guides, so that it 



