242 Sir D. Brewster on the Influence of successive. 



Mrs. Griffiths. The brilliancy of the spectrum thus pro- 

 duced, and the beauty of its colours, exceed any optical phe- 

 nomenon which I have witnessed, and so dazzling is its effect 

 that the eye is soon obliged to withdraw itself from its over- 

 powering influence. 



The very same phenomena may be seen by looking at the 

 sun through the distended fingers when they are made to 

 move backwards and forwards, or rather from right to left, 

 and from left to right, in front of the eye. 



The colours of the spectrum above described have their 

 origin in the red light transmitted through the eyelids, the 

 green tints being the accidental or complementary colour of 

 the red; but the phenomenon may be seen in a great measure 

 without colour by opening the eye, and interposing between 

 the eye and the sun any white transparent ground, such as 

 thin white paper or ground glass, or by directing the eye im- 

 mediately to a bright sky, or to the ground when covered with 

 snow. In these different forms of the experiment the effect 

 varies greatly with the intensity of the light and the state of 

 the eye, but the following general description of the pheno- 

 mena will be found tolerably correct. In order to make the 

 light produce a series of successive impulses on the retina, and 

 on the same parts of it, I look through the openings of the 

 revolving disc of the phenakistiscope with one eye, and fix it 

 steadily upon the same point of the luminous ground. 



When the disc revolves with great velocity, a very faint 

 and uniform light is seen over the whole luminous surface. 

 As the velocity diminishes, the light becomes less uniform, and 

 a flickering or wavering motion commences. Patches of a 

 bluish purple colour appear in different parts of the field, form- 

 ing a sort of network, the intersections between the meshes 

 of which are of a faint lemon-yellow colour, the accidental co- 

 lour of the bluish purple. The pattern of this network is 

 related to the centre or point on which the eye is fixed, and 

 seems to belong either to a hexagonal or octagonal division cf 

 the circle. The centre of the pattern, corresponding to the 

 foramen centrale, is a square or lozenge, one of whose diago- 

 nals is vertical ; but as the differently coloured patches or 

 elements of the pattern are constantly changing their colour, 

 their intensity of light, and even their form, owing to the un- 

 steadiness of the eye and the revolving disc, 1 have never been 

 able to draw the pattern, or to trace how the patches or inter- 

 stices of the net-work spring from the sides and angles of the 

 central lozenge. That the reticulated structure is related to 

 this central square or to the central foramen of the eye is 

 unquestionable ; and I have no doubt that observers who have 



