100 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
tained (as elsewhere has been shown and as de Rlieita also has said) by 
the aid of a pair of compasses and a mirror. This distance serves for 
adjusting the first ocular lenses, since in the binocular tube the two 
other lenses nearest the eye must be similarly placed to each other— 
e. g. if the centres of the pupils of any person be distant A B, 22/100 
ped. Rom. (fig. 3, I.), at this distance also from each other must the other 
first lenses be, ab and c d, as is shown by the figure, so that the centres 
of the lenses a b and c d should meet at the points A and B.* 
‘ Let us suppose one simple tube be constructed with a convex ocular 
lens of very great aperture, which shall be greater than the distance of 
the two eyes, generally speaking it will be impossible, by using one 
convex ocular lens, however large, for an image radiating from the tube 
to affect both eyes alike, for if the image radiating from the tube through 
one lens could affect both eyes alike, both eyes must be at the point of 
contact ; but this is impossible, since both eyes cannot be at the same 
point at the same time, and thus all parts and points of the image cannot 
be transmitted to the same parts and points ox both eyes at once. I 
have said generally speaking, because if one look at a mirror of very 
obtuse convexity and at a great distance from the eyes, perhaps some- 
thing can be effected : but it is of little practical use, for the objective lens 
must be of extremely long diameter to represent the object sufficiently 
large and near, so the tube would be obliged to be of enormous length. 
‘ III. A binocular tube can be constructed with one objective lens 
when the image thrown from the objective lens can extend itself to a 
sufficient distance in the base of the divergence and thence again radiate 
through ocular lenses to both eyes. Thus the object C P D (fig. II.) 
radiating through the objective lens A B, forms the image E F ; but the 
rays of the image, digressing from E, reach the eye N through the ocular 
lenses GIL, and from F reach the eye O through the lenses HK M. 
Thus a binocular tube can be constructed with one objective lens A B ; 
but by this method the rays from the object reach the eye after being 
greatly refracted and diverging very far from the axis P Q of the objec- 
tive lens, so that the image is never clear and distinct. Moreover, 
both eyes cannot see the whole of one object, nor is the same part of 
the object seen by both eyes, though more of the object can be seen by 
moving it about, but it w'ill always be confused. Hence a tube thus 
constructed is never of much use. 
‘ IV. Even if one large lens be taken having two apertures at exactly 
the proper distance apart, and both eyes look through these apertures at 
a single object, and thus see one and the same thing (which, however, 
seems impossible), nevertheless a perfect binocular tube cannot be 
formed, because, just as the radiation from the object comes to both eyes 
by rays remote from the axis, and hence more refracted, so will the image 
possessed in the eye be indistinct and confused. 
‘ V. The best construction of the binocular tube is made by two tele- 
scopes exactly alike, so fitted to both eyes that the optical axis passes 
through one and the same object (figs. III., IV., VI., VII. VIII.). The 
* The plate is reduced one-half from pencil tracings. The letters on fig. I. were 
accidentally omitted. A B are the ends of the straight line, ab, be the diameters of 
the circles on ttie same line prolonged. The circles represent the eye-lenses to be 
just as far apart as the observer’s eyes. 
