480 Capt. Abney and Lt.-Col. Festing. [Jan. 25, 



34. This points to something anomalous in the centering of the 

 eye surfaces. The amount and direction of the eccentricity in my 

 eyes as shown by these means are in somewhat remarkable correspon- 

 dence with those obtained by the entirely different method of Helm- 

 holtz (" Phys. Opt.," pp. 86, 87) - I may add below the angles for my 

 eyes corresponding to numbers given by Helmholtz, loc. cit. : — 



Light came from 

 Nasal side. Temporal side. 



Right eye 9° 27' .... 3° 49' 



Left eye 7 Q 36' .... 7° 36' 



35. One more observation of some interest I may record. With the 

 sun as source of light and with a fixed direction of vision (I have 

 generally taken that direction in which the ghost comes into the line 

 of direct vision, so as to avoid the constant tendency of the eye to turn 

 towards any object of interest in the field), the ghost passes through 

 a series of forms as the state of accommodation alters. There are 

 three marked forms, a horizontal bar with a near focus ; a circle for a 

 slightly more remote focus ; and a vertical bar for long distance. 

 There may be signs of a double astigmatism ; but as they are the only 

 signs that I have had of such defect in my eyesight, I have rejected 

 this explanation in favour of the equally simple one, that we have 

 here the focal lines and circle of least confusion such as are observed 

 in oblique reflexion by a concave mirror. 



The explanation suggested in this paper is attended with difficul- 

 ties, the greatest of which is perhaps that referred to in § 27. It is, 

 however, the one which on the whole explains best the greatest 

 number of the facts to be accounted for. The calculations made on 

 various points mentioned above are based on measurements given by 

 Helmholtz for his schematic eye. It seems likely that individual 

 variations from the normal eye should be such as to determine the 

 visibility of the ghost ; but I have found as yet no obvious relation in 

 about fifteen cases between the power to see the ghost and the kind 

 of sight, as defined by the ordinary terms, long and short sighted- 

 ness. Two cases in which I have notably failed to show the ghost, 

 are opposed in this respect, one having short sight, the other normal ; 

 whilst of those to whom the ghost is visible there are as many short 

 sighted as long sighted. 



III. "Note on the Absorption Spectrum of Iodine in Solution in 

 Carbon Disulphide." By Captain Abxey, R.E., F.R.S., and 

 Lieut.-Colonel Festing, R.E. Received January 18, 1883. 



In 1876, Sir John Conroy (" Proc. Roy. Soc," vol. 25, p. 46), 

 described the absorption spectrum of a solution of iodine in carbon 



