1873.] 



Dr. J. Jago on Visible Direction. 



213 



Seeing, then, that in the case of sodic sulphate, which is said to be 

 always present in the air of rooms, and, according to MM. Grernez and 

 Viollette, even in that of the country, the chances are that it is most 

 likely to be present either in the effloresced condition or in solution, 

 and equally non-nuclear in both, I cannot help thinking that too much 

 importance has been given to this part of the subject ; for if it be 

 true, we are reduced to the dilemma, pointed out by M. Jeannel*, that 

 there must be floating in the air specimens of all kinds of salts that form 

 supersaturated solutions and crystallize by the introduction of a solid 

 nucleus ; whereas there are some such salts which cannot exist in the pre- 

 sence of the oxygen or of the ammonia of the air. 



II. " Visible Direction: being an Elementary Contribution to 

 the Study of Monocular and Binocular Vision." By James 

 Jago, M.D. Oxon., A.B. Cantab., F.R.S. Received February 

 12, 1873. 



(Abstract.) 



It is a well-known fact that when the eye has been displaced in its 

 socket, as, for instance, by the tip of the finger applied to the eyeball 

 through the eyelid, all objects seen by it deviate from their true direc- 

 tions ; and the author's mode of proceeding in this paper is to inquire 

 whether visual deviations that may be observed in arbitrary, but 

 methodically devised, displacements of the eyeball in its socket follow 

 any law, and then to consider how far the results thus derived are con- 

 formable with other monocular and binocular experiences, and how far 

 they may be available in the explanation of certain phenomena that have 

 been deemed anomalous in physiological optics. 



Having pointed out means by which the ball may be easily displaced in 

 any direction, he draws attention to the fact that, when by such means 

 the apparent directions of objects seen by the eye are made to deviate from 

 their true directions through fully 30°, the orbital muscles so fully retain 

 their command over the movements of the eyeball, that that point in the 

 visual field which was painted on the point of direct sight in the centre 

 of the foramen centrale retinae still continues to be there painted. He 

 shows this to happen whatever be the direction in which the eyeball is 

 displaced in its orbit. 



This fact being a fundamental one in the inquiry he has in hand, he puts 

 it to nicer tests still. 



He adjusts the two eyes, when equally displaced so as to cause objects 

 to deviate greatly from their true directions, to look awhile at the top of 

 a high object in the open air, and having obtained a strong spectrum of 

 this object in the retinae, he, with the released eyes, looks at an appro- 



* Ann. de Ch. et de Ph. 4th ser. vol. vi. p. 166. 



