OCULAE SPECTRA. 21 



masses of coloured light which float before the 

 eye may be moulded by the same power into 

 those fantastic and natural shapes, which so 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 in- 

 fluence of the mind, which imparts a new cha- 

 racter 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 accidental colours. If we cut a figure 

 out of read 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 differ- 

 ently coloured figures we shall observe differently 

 coloured spectra, as in the following table : 



COLOUR OF THB COLOUR OF THB 



ORIGINAL FIGURES. SPECTRAL FIGURES. 



Red, Bluish-green. 



Orange, Blue. 



Yellow, Indigo. 



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 B piece of 

 canvas over the figure to cover it, for I could not bear to 

 have it always before my eyes." 



