182 Scientific Proceedings, Royal Dublin Society. 
disadvantage of having to mount the two pictures on separate 
cardbourds, and (consequently on this) the necessity of adjusting 
each pair of pictures carefully, caused this stereoscope to be com- 
pletely rejected for the more convenient, more portable, and 
handier, though by no means so perfect instrument, the Brewster 
Lenticular Stereoscope, as that in general use is termed—(Fig. 3, 
Plate 1). 
In this stereoscope the pictures are mounted side by side on 
one piece of cardboard, and the optical arrangement consists in a 
pair of half convex lenses, or what is equivalent to the same, whole 
lenses, mounted at a greater distance apart than the eyes of an 
ordinarily constituted being. This arrangement (too well known 
to need description) enables the observer, without trouble or 
strain, to cause the images to coalesce, and produces the stereo- 
scopic effect, provided the instrument be well adopted to the 
observer's eyes. 
This may, perhaps, be the best place to say a few words on a 
matter about which there was some considerable amount of dis- 
pute some five or six years ago. As two of those who took part 
in the dispute have since been called away, I do not propose to 
say more than is necessary to set the matter right should anyone 
have doubts. 
It was argued very plausibly by one, whose profuse and con- 
tinual writings on such subjects gave his dicta on these matters 
considerable and just weight, that no prism power at all was 
necessary or even desirable in the Brewster Stereoscope, and that, 
inasmuch as the axes of our eyes were parallel, or nearly so, in 
looking at some distant object in the landscape, so a truer repre- 
sentation of the object would be obtained if the axes of our eyes 
were parallel when looking at the photos. In fact, if they were 
so, we would havea true representation of the object while, if we 
used such optical contrivances as would render the axes of the 
eyes convergent when looking through the stereoscope, we would 
have a representation, not of the object, but of a small model of 
the object. 
It may be quite true that this arrangement would, in one 
sense, give a true and accurate representation of the object, but in, 
another sense, it gives quite the reverse, for while, to an inex- 
perienced eye, the view of the object itself is pleasant, and free 
