GENERAL VIEWS. 53 



every object appeared to them as red as scarlet. 

 Now this red light was probably nothing more 

 than the red phosphorescence produced by the 

 pressure of the blood-vessels on the retina, and 

 analogous to the masses of blue, green, yellow, 

 and red light, which have been already mentioned 

 as produced by a, similar pressure in headaches, 

 arising from a disordered state of the digestive 

 organs. 



Were we to analyse the various phenomena of 

 spectral illusions, we should discover many cir- 

 cumstances favourable to these views. In those 

 seen by Nicolai, the individual figures were 

 always somewhat paler than natural objects. 

 They sometimes grew more and more indistinct, 

 and became perfectly white ; and, to use his own 

 words, "he could always distinguish with the 

 greatest precision phantasms from phenomena." 

 Nicolai sometimes saw the spectres when his 

 eyes were shut, and sometimes they were thus 

 made to disappear, effects perfectly identical 

 with those which arise from the impressions of 

 very luminous objects. Sometimes the figures 

 vanished entirely, and at other times only pieces 

 of them disappeared, exactly conformable to what 

 takes place with objects seen by indirect vision, 

 which most of those figures must necessarily 

 have been. 



Among the peculiarities of spectral illusions, 

 there is one which merits particular attention, 

 namely, that they seem to cover or conceal 

 objects immediately beyond them. It is this 

 circumstance more than any other which gives 

 them the character of reality, and at first sight it 

 seems difficult of explanation. The distinctness 

 of any impression on the retina is entirely inde- 



