184 Scientific Proceedings, Royal Dublin Society. 
tion of the eyes, and hence it is, probably, that stereoscopes are 
made generally with too little divergent power, the makers have 
got so accustomed to diverging their eyes, that, probably, they 
require little or no divergent power at all. 
I can, myself, without almost wishing it or thinking of it take 
two stereoscopic pictures large or small and diverging my eyes look 
at the left picture with the left eye, and right with the right, and 
now instantaneously change, and converging my eyes look at the 
right picture with tke left eye, and the left picture with the right 
eye. I can (probably by long practice) converge and diverge 
my optic axes twenty times a minute, but because I can do this 
and thus do without a stereoscope at all or with a bad one, this 
is no reason why we should not give the best possible help to 
those who are not so practised in the matter. 
Now, I may here briefly mention a few of the disadvantages 
these stereoscopes labour under. 
The disadvantages of the Wheatstone need hardly be touched 
upon. Some of them have already been mentioned, and its use has 
been long since practically discontinued. 
The disadvantages of the Brewster Stereoscope are :— 
1. Distortion of images to a certain extent. 
2. Chromatic fringes on lateral objects. 
3. Limit of its application to pictures whose breadth is not 
greater than the distance between the human eyes. 
4. As a consequence of the last, the pictures are almost always 
made a most unartistic shape (viz., square). 
5. Many persons have a great difficulty in combining the images 
as seen in the Brewster Stereoscope. 
This probably arises from the fact that different individuals re- 
quire different adjustments of the instrument, not only for focus 
(to suit long and short sight), but also in the amount of apparent 
displacement of the images necessary to make them overlap and 
combine, and also that persons differing from one another in the 
distances asunder of their eyes will, in the ordinary form of the 
Brewster Stereoscope, have, of course, a different amount of dis- 
placing power supplied to them by the same instrument according 
to whether they be looking more nearly through the centre of 
edge of the lenses. Consequently, if we wish to construct a stereo- 
scope which shall be the best possible for any one individual, due 
