122 Scientific Proceedings, Royal Dublin Society. 
blind spot or punctum caecum was discovered by Mariotte upwards 
of two hundred years ago, and is therefore termed “ia tache de 
Mariotte”’ in France. As the optic nerve enters the eye on the 
jnner or nasal side of the yellow spot, and a little below the level 
of the latter, in the visual field the blind spot appears to be slightly 
above the level of the yellow spot, and on the outer side of it, 
owing to the inversion of the projected image. (Expl. Pl. VII.) 
The distance which separates the centre of the blind spot 
from the centre of the yellow spot—the point of fixation of the 
eye—has been the subject of numerous experiments by Listing, 
Helmholtz, Dobrowolsky, Mauthner, Snellen and Landolt, and 
others. The general results of these measurements show that, 
from the point of fixation to the centre of the blind spot, an 
angle of 15° is subtended in normal or emmetropic eyes: in the 
case of myopic eyes the angle may be as small as 11°, and in 
hypermetropic eyes as large as 19°. This difference obviously 
arises from the fact that the eyeball is longer in myopic and 
shorter in hypermetropic than in normal eyes. The “near 
distance, D, between the two points (the centres of the macula and 
of the papilla) depends upon this angie, a, and on the distance of 
the nodal point, «, from the retina, or 
- a 
D = 2k sin one 
If « be taken as 16:6 mm. for emmetropic or normal eyes, as 
Landolt, Snellen, and Hirschberg have found, then the angle 15° 
gives D as = 43 mm. Ii « be taken as 16, then the distance 
becomes a little less, viz. 4:14 mm. Measurements made upon 
extirpated eyes correspond to this value.' 
I am indebted to the eminent London oculist, Dr. Lawford, 
for some measurements of this distance in microscopic slides of 
the retina (section through the fovea). Dr. Lawford finds the 
mean of three different cases to be 4 mm.; but he hopes to give 
‘me some further measurements immediately after removal of 
the eye. 
The dimensions of the blind spot have been found in like 
1 According to Snellen and Landolt, Zraité compléte d’ Ophthalmologie, vol.i., 
p- 621. A full discussion of the blind spot is given in this work. 
