Dr. H. Airy on a distinct 



[Feb. 17, 



the field of view, because the nasal half of the field of view of either eye is 

 more limited, and vision there is less distinct than on the temporal side. 



Looking at any surface of uniform colour, the cloud partakes of the 

 general hue of the field on which it lies, and shows little that is charac- 

 teristic except its bright margin, tremor, and boiling. 



Against bright light a faint shadowy curved cloud is seen, with bright 

 margin, tremor and boiling, and slight colour. 



Against dark shade the cloud is seen to show faint light. 



"When part of the cloud is seen against dark shade and part against 

 bright light, the boundary between the light and shade is wholly obliterated. 



Viewed in the dark, the cloud presents inherent luminosity, especially at 

 the margin. Its various colours are seen as well in dark as in light. 



The cloud spreads outwards in horseshoe shape till it reaches the out- 

 skirts of the field of view, and fades away after great boiling and turbu- 

 lence. The lower arm is the first to fade, then the middle, and finally the 

 upper arm, which remains pointing to the centre of the field to the very 

 last. 



The climax is reached in about twenty-five minutes from the first be- 

 ginning. The whole duration of the attack is just half an hour. 



Often, midway in the attack, a fresh germ of blindness arises near the 

 birthplace of the first, but always proves abortive unless it takes root on 

 the opposite side, when a second attack may develope itself immediately 

 after the first. 



This half-bindness is followed by oppressive headache, lasting many 

 hours. 



From the resemblance of the angular margin of the cloud to a fortified 

 wall " with salient and reentering angles, bastions, and ravelins " (to use 

 Sir John Herschel's words), the author ventures to suggest the name 

 Teichopsia for this striking form of transient half-blindness. 



Among the circumstances that have seemed to favour an attack may be 

 mentioned sudden change of air and living, over-exercise, and insivfficient 

 sleep. The attack has sometimes been nocturnal. 



The most usual position of the germ of blindness is 3° or 4° below, and 

 3° or 4° to the left of the centre of vision. 



In one or two cases, after reaching a certain stage, the cloud has parted 

 in the middle, and died away without ripening. 



The cloud, whether developed in the right or the left half of the field, 

 has never (the author believes) transgressed the vertical median line. 



Lately, one or two attacks have been followed by a slight disturbance of 

 hearing. 



Of three cases coming under the author's immediate observation, in 

 one these attacks have been very frequent, from an early age to middle 

 life. The bastioned outline is always present, with more or less colour. 

 Formerly the attendant headache used to be very severe, accompanied with 

 prolonged vomiting. Latterly the blindness has been more oppressive than 



