Improvements in the Stereoscope. 185 
regard must be paid to the particular angle of convergence that 
will best suit that individual, and the distances asunder of his eyes. 
Again, the human eyes have a wonderful facility for altering 
the relative direction of their axes in a horizontal plane (by a mere 
effort of will) ; hence many people can see with a stereoscope not 
at all suited to their sight or without a stereoscope at all; but if 
they do, it is with an effort which, if long continued, becomes 
painful. 
Glancing back over this list of disadvantages we can, I think, 
easily pick out that one which has caused this instrument to de- 
cline in favour so much as to be now considered almost old- 
fashioned. It is evidently No. 3—that of the limit of its adapti- 
bility to pictures no larger than the ordinary little 2? stereoscopes 
of which people are so sick and tired. 
I shall now just mention, as far as possible, in this chronological 
order, a few of the modifications proposed in these forms of in- 
struments. 
About the year 1856 my father (the late Thomas Grubb, F.R.s.), 
finding great difficulty in lighting the two pictures of the Wheat- 
stone Stereoscope, altered the position of the pictures and angles 
of the reflectors till they assumed the form shown in Fig. 4, Plate 
10; this did not impair the perfection of the result, and had some 
considerable advantages as to lighting, &e. 
In 1860, and again in 1873, Mr. Thomas Sutton brought out a 
stereoscope, shown in Fig. 5, Plate 10, which appears to be just 
a carrying out of the same idea to a still greater extent. 
In 1873 I mentioned to one of the editors of the British Journal 
of Photography a plan I had used for viewing large pictures 
stereoscopically by revising the prisms of an ordinary Brewster 
stereoscope (see Fig. 6, Plate 10) ; this was published in the Photo- 
graphic Journals, and caused much discussion. 
It was, however, objected to on the ground that the high con- 
veyance of the optic axes caused the picture to assume too much 
the appearance of a model. I had never put this forward as a 
perfect instrument ; but as soon as this discussion opened I pub- 
lished another form, Fig. 7, Plate 10, which appeared to me, ex- 
cepting in the matter of expense, very unexceptionable indeed. 
The form I have to consider further on, however, seems to 
possess some advantages over this, 
