68 Scientific Proceedings, Royal Dublin Society. 
profession, it has its limitations, especially when there is any 
opacity in the anterior parts of the eye; and I imagine that cases 
may frequently occur when the Entoptiscope can alone enable a 
correct diagnosis to be made by the ophthalmic surgeon. 
Tn order to save calculation, the distance from the glass stage, 
on which the image is projected, to the pin-hole aperture, can be 
made five or ten times the distance of the aperture from the pupil 
Fig. 6. 
of the eye; the linear magnification is thus five or tenfold; and 
the exact area of the entoptic obscurity is accordingly 25 or 100 
times less than the area of the image drawn by the patient. 
In ordinary vision, the magnitude of the image of an object on 
the retina is to the size of the object in the ratio of the distance of 
the nodal point of the eye from the retina (viz., 16 mm.) to that of 
the distance of the nodal point from the object. This law also 
holds true of entoptic vision when the shadows on the retina can 
be seen by a general illumination of the eye. Helmholtz, Donders, 
and all recent authorities on physiological optics, so far as I know, 
