The Astronomer Royal on Hemiopsy. 19 



of the salt, instead of saying that the zinc displaces the lead from 

 its combination with acetion. Again, when silver nitrate and 

 sodic chloride, each in solution, are mixed, the chlorine and the 

 nitrion change places, 62 of nitrion, N 2 O 5 , being equivalent in 

 saturating power to 35*5 of chlorine. Let any one try to explain 

 this decomposition in our ordinary chemical language, and he 

 will see the advantage gained by adopting the use of some such 

 terms as those now proposed. 



King's College, London, 

 June 10, 1865. 



III. The Astronomer Royal on Hemiopsy. 



To the Editors of the Philosophical Magazine and Journal. 



Royal Observatory, Greenwich, S.E., 

 Gentlemen, June 5, 1865. 



I REMARK, in a late Number of the Philosophical Magazine, 

 a paper " On Hemiopsy " by Sir David Brewster, in which 

 Sir David has given the results of the experience of that singular 

 malady by. Dr. Wollaston and by himself. I have myself been 

 frequently attacked by it, certainly not fewer than twenty times, 

 probably much oftener ; and I am acquainted with two persons 

 who have suffered from it, one of them at least a hundred times. 

 From the information of my friends, and from my own experi- 

 ence, I am able to supply an account of some features of the 

 malady which appear to have escaped the notice of Dr. Wollaston 

 and Sir David Brewster. 



One of my friends attributes each acces of the disease to men- 

 tal anxiety or overcrowding of business. In my own ease, I 

 have never been able to refer it distinctly to any antecedent 

 cause. In a few instances I have known it to follow that state 

 of the brain or eye in which a sudden pressure of the breath 

 causes the appearance of numerous spherical globules floating 

 across the field of view. 



I discover the beginning of the attack by a little indistinctness 

 in some object at which I am looking directly ; and I believe 

 the locality of this indistinctness upon the retina to be, not the 

 place of entrance of the optic nerve, but the centre of the usual 

 field of vision. Very soon I perceive that the indistinctness is 

 caused by the image being crossed by short lines which change 

 their direction and place. In a little time the disease takes its 

 normal type, and presents successively the appearances shown in 

 the following diagram. In drawing this, I have supposed that 

 the principal obscuration of objects is apparently on the left side; 

 by reversing the figure, left to right, the appearances will be 



C2 



