106 LETTERS ON NATURAL MAGIC. 



light which float before the eye may be moulded by the 

 same power into those fantastic and natural shapes wliich 

 20 often haunt the couch of the invalid, even when the 

 mind retains its energy, and is conscious of the illusion 

 under which it labours. In other cases, temporary 

 blindness is produced by pressure upon the optic nerve, 

 or upon the retina, and under the excitation of fever or 

 delirium, when the physical cause which produces spectral 

 forms is at its height, there is superadded a powerful 

 influence of the mind, which imparts a new character to 

 the phantasms of the senses. 



In order to complete the history of the illusions which 

 originate in the eye, it will be necessary to give some 

 account of the phenomena called ocular spectra, or acci- 

 dental colours. If we cut a figure out of red paper, and 

 placing it on a sheet of white paper, view it steadily for 

 some seconds with one or both eyes fixed on a particular 

 part of it, we shall observe the red colour to become less 

 brilliant. If we then turn the eye from the red figure 

 upon the white paper, we shall see a distinct green figure, 

 which is the spectrum, or accidental colour of the red 

 figure. With differently coloured figures we shall ob- 



" One remarkable thing was, one day as we mended a sail, it 

 being a veiy thin one, after laying it upon deck 'in folds, I took the 

 tar-brush and tarred it over in the places which I thought needed 

 to be strengthened. But when we hoisted it up, I was astonished 

 to see that the tar I had put upon it represented a gallows and a 

 man under it without a head. The head was lying beside him. He 

 was complete, body, thighs, legs, arms, and in every shape like a 

 man. Now, I oftentimes made remarks upon it, and repeated them 

 to the others. I always said to them all, you may depend upon it 

 that something will happen. I afterwards took down the sail 

 on a calm day, and sewed a piece of canvas over the figure to cover 

 it, for I could not bear to have it always before my eyes." 



