470 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
tion of such an instrument. A sketch of the history of Greenough’s 
endeavour to solve the problem is also given. In analysing the con- 
ditions of orthomorphic vision, Greenough’s equation is arrived at, viz. 
Y = -=■ , where Y = the linear magnification of the single Microscope ; 
Cl 
D = distance of the observer’s eyes ; d = distance of the light entrances 
of both Microscopes. Another condition can be thus expressed : — 
“ The image must in all its parts in each Microscope-tube appear from 
the point of sight under the same angular distance as the object from 
the focus of the primary rays or still more simply, “ Entrance-pupils 
and exit-pupils of the Microscope must be on the same points of 
junction ” ( Knotenpunhte ). 
In order to investigate this last condition, one must realise that 
corporeal images are never seen as such, but construct themselves by 
an unknown psychic process out of two different plane retinal images. 
Let us take three points a, b, c (fig. 75) not lying in a straight line as the 
Fig. 76. 
object, and imagine two Microscopes M lt M 2 inclined to one another at 
an angle of about 14°, and so directed towards the group of points that 
the picture of all three falls in the field of view of both Microscopes. 
Let Z, r, be the “ entrance-pupils ” of both Microscopes in Abbe’s sense, 
i.e. the focus of the image-forming pencil of rays, and consequently the 
perspective projection centre for representation. Let c h c r be the pro- 
jections of the point c on the line a b produced backwards from Z and r. 
Then the working of the Microscope confines itself to projecting in all 
its parts an equally magnified reproduction of a b c, and a b c r . 
The determination of the visual angles under which the images of 
a c x b and ac r b, i.e. A L C L B L and A R C R B R , must be presented to both 
eyes in the case of normal accommodation, requires that the optical axes 
should virtually intersect in a point C, whose distances in space from 
A and B stand to one another in the same ratio as those of c from a and b. 
This is satisfied only when the angles, under which the image-points 
A L , B L , C l , A r , B r , C r appear simultaneously to the eye, are equal to 
those under which the corresponding object-points a, b, c from Z and r 
respectively appear. Increase of this angle would be in stereoscopic 
