440 
Proceedings of the Royal Society 
tliat tlie principal optical centre of the dioptric apparatus is more 
than 7 mm. anterior to the centre of rotation. In the following 
table I have estimated the degree of error for each of conver- 
gence with distant vision. It is seen to be very trifling, and to reach 
half a millimetre only with 4° of convergence in emmetropia. In 
myopia the error would he greater. 
0° 30' 
‘06465 mm. 
3° 30' 
■4221 mm. 
r 
'12635 mm. 
4° 
■5050 mm. 
r 30' 
T895 mm. 
4° 30' 
*568 mm. 
2° 
'2527 mm. 
5° 
•631 mm. 
2° 30' 
'3158 mm. 
7° 56' 
1 mm. 
3° 
■3789 mm. 
error is 
avoided by the practice 
of shutting 
each eye in 
It may be suggested that the eyes tend to take the position in which 
their optic axes are parallel, rather than their visual axes, but this 
would not account for the convergence of myopes in which the two 
axes coincide ; and in my own case, convergence in spite of hyper- 
metropia is almost certainly less than the angle between the visual 
axes by two or three degrees at least. The two conditions so far 
seem to be quite independent. 
2. On the Formation of Small Clear Spaces in Dusty Air. 
By Mr John Aitken. 
{Abstract ) 
In the introduction a few remarks are made on the growing 
interest in everything connected with dust, whether it be the 
organic germs floating in the air, or the inorganic particles that 
pollute our atmosphere. Professor Tyndall’s observations on the 
dark plane seen over a hot wire"^ are referred to. Lord Eayleigh’s 
recent discovery of the dark plane formed under a cold bodyf is 
described, and attention called to Dr Lodge’s experiments, detailed 
in a letter to Nature, vol, xxviii. p. 297. 
* Essays on the Floating Matter in the Air, p. 5, Longmans, Green, & C 
1831, 
i Nature, vol, xxviii. p. 13^ . 
