I do wonder if BarbaraB has hit on something re allergies,( see her post below) because of this quote from Letter 18:

My eyes have been very indifferent since it was written, but are now getting better once more; keeping them so many hours open on Thursday night, as well as the dust of the ball-room, injured them a good deal. I use them as little as I can, but you know, and Elizabeth knows, and everybody who ever had weak eyes knows, how delightful it is to hurt them by employment, against the advice and entreaty of all one's friends