Improvements in the Stereoscope. 189 
images viewed by looking in horizontally (Fig. 2, Plate 11). 3. It 
may be made with a pair of prisms, achromatic by preference, 
turned, of course, though 90° as respects the Brewster stereo- 
scope, and this form can be made of the shape and character of 
any of the Brewster type. 4. If it be thought a disadvantage 
that the pictures should be reversed, as they must be in this or 
any single reflecting stereoscope, a pair of prisms or reflectors can 
be used that will reflect the light twice, and thus erect the pic- 
tures again (fig. 3, Plate 11). 
Now, as to the objections that will be urged against this stereo- 
scope, it is as well to anticipate them. It will be said that if the 
pictures be mounted on the one card each picture cannot be 
viewed from its proper point of sight, and, therefore, we will not 
receive a correct impression of the subject. Well, my answer to 
this is, first, as to the vertical direction, that nothing is easier 
than to mount the pictures on two pieces of cardboard jointed to- 
gether, and have the table of the stereoscope not flat, but of such 
form as will cause the folded cardboard to lie not quite open, just 
so much as will bring matters right in that direction (as in Fig. 1, 
Plate 11); though I would remark that for views of such a pro- 
portion as I consider best suited to this stereoscope this is almost 
unnecessary. 
As regards the other direction, viz., horizontally, the right eye 
is, of course not opposite to, but about one and a-quarter inches 
to the right of, the centre of the picture it views, and the left 
about one and a-quarter inch to the left of the centre of the picture 
it views. This would be running matters to extremities to com- 
plain of, except in very small pictures ; but I am of opinion, from 
careful trial, that it is easier on the eyes that it should be so, for 
our optical axes are then exactly in the condition they would be 
in looking without the stereoscope at an object at same distance. 
One great charm of this stereoscope to the experimentalist is 
that all these experiments can be tried in it so readily. Another 
objection—that the pictures will be reversed—can be met by the 
adoption of the fourth form mentioned above; but Iam by no 
means sure that this is an objection at all, for it would appear 
that in some of the more favoured carbon processes in vogue now 
trouble is found in transferring a second time, so as not to have 
the picture reversed. If this be so it will only be necessary to 
