of Edinburgh^ Session 1883-84 
437 
and fix a small spirit-level parallel to tlie line which joins them. 
The results of these experiments I must defer. It remains to 
Fig. 9. 
Position of two eyes when looking at the two holes, as in fig, 8 (exaggeruted). 
V = visual axis of each eye. The left eye is looking direct at the left hole, 
hut the right is somewhat convergent, so the image falls on x instead of on z, 
and is referred in consequence outwards. 
Centre of Eotation, c. 177 mm. behind centre of optic axis (Bonders), 
Principal optical centre p. 
The optic axis o” (left eye) is seen not to coincide with the visual axis V. 
estimate the exact degree of convergence which is naturally asso- 
ciated with negative accommodation. 
To this end I have made use of a kind of camera devised for 
another purpose, and find with my own eyes that convergence 
