OPTICS. 



a faint red border. When he opened 

 his eye and turned it to a white ground, 

 the image of the sun was brownish red, 

 and its border sky blue. With his 

 eye again shut, the image appeared 

 green, and the border a red diiferent 

 from the last. On opening his eye, and 

 turning it to a white ground as before, 

 the image was more red than formerly, 

 and the border a brighter sky blue. 

 His eye being again shut, the image was 



freen, approaching to sky blue, and the 

 order a red still differing from ^the 

 former. When his eye was again open- 

 ed upon a white ground, the image was 

 still red, and its border sky blue, but 

 with different shades from the last. At 

 the end of four or five minutes, when 

 his eye was shut, the image was a fine 

 sky blue, and the border a brilliant red; 

 and upon opening his eye as formerly 

 upon a white ground, the image was a 

 brilliant red, and the border &fine sky 

 blue.* 



Experiments of a similar kind were 

 made by Dr. Brewster, by looking at a 

 brilliant image of the sun's disk formed 

 by a concave reflector. With his right 

 eye tied up, he viewed this luminous 

 disk with the left through a blackened 

 tube, to prevent any extraneous light 

 from falling upon the retina. When 

 the retina was highly excited by this 

 intense light, he turned his left, eye to 

 a white ground, and perceived the fol- 

 lowing spectra by alternately opening 

 and shutting the eye. 



Spectra with the left Eye open. Spectra with the left Eye shut. 



1. Pink surrounded with Green, Green. 



2. Orange mixed with Pink Blue. 



3. Yellowish Brown Bluish Pink. 



4. Yellow Lighter Blue. 



5. Pure Red Sky Blue. 



6. Orange Indigo. 



These spectra were always surround- 

 ed with a ring of the accidental colour, t 



The phenomena above described, 

 prove that the common theory of acci- 

 dental colours is true only with weak 

 degrees of light, for in the preceding 

 experiments the spectrum ought to 

 have been black. 



If when one of these spectra is visi- 

 ble we press the eye to one side, the 

 spectrum will appear to be absolutely 

 immovable, if the experiment is not 

 made with much attention ; and upon 

 this imperfect observation, Dr. Wells 

 and others have founded strange theo- 



* See Nov. Comment. Pctrop. torn. x. p 283. 

 t Edinburgh Encyclopa-dia, Art. Accidental Co- 

 lours, vol. i. p. 90. 



ries. It will be found, however, by 

 pressing both the eyes at once, and by 

 due attention to their corresponding 

 motions, that the spectrum does move, 

 and that it is seen by the eye in the same 

 manner as if it were the image of an 

 external object, conformably to the law 

 of visible direction.* 



By means of pressure upon the eye- 

 ball, ocular spectra may be produced ; 

 and when spectra produced by exter- 

 nal impressions of light are seen by the 

 eye, their colours are changed by pres- 

 sure on the eyeball. The pressure of 

 the blood vessels on the back of the 

 eye often produces spectra, in particu- 

 lar states of the stomach. In slight 

 affections, these spectra are floating 

 masses of blue light, which appear and 

 disappear in succession ; but in severe 

 ones, they become green, and sometimes 

 rise to yellow. Hence it follows, that 

 pressure upon the retina creates the 

 sensation of light and colours. 



1 5 . Colours produced by the unequal 

 action of light upon the eyes. If we 

 hold a slip of white paper vertically 

 about a foot from the eye, and direct 

 both eyes to an object at some distance 

 beyond it, so as to see the slip of paper 

 double, then when a candle is brought 

 near the right eye, so as to act strongly 

 upon it, while the left eye is protected 

 from its light, the left-hand slip of paper 

 will be of a tolerably bright green 

 colour, while the right-hand slip of paper 

 seen by the left eye will be of a red 

 colour. If the one image overlaps the 

 other, the colour of the overlapping 

 parts will be white, arising from a mix- 

 ture of the complementary red and 

 green. When equal candles are held 

 equally near each eye, each of the ima- 

 ges of the slip of paper is white. If 

 when the paper is seen red and green. 

 by holding the candle to the right eye, 

 we quickly take it to the left eye, we 

 shall find that the left image of the slip 

 of paper gradually changes from green 

 to red,, and the right one from red to 

 green, both of them having the same 

 tint during the time that the change is 

 going on. This beautiful experiment 

 was first made by Mr. Smith, surgeon 

 in Kinguissie.t and seems to confirm 

 the observation made by Dr. Brewster, 

 in the article on accidental colours 

 already quoted, that in certain highly 

 excited states of one eye, the reverse im- 



* F.dinluryh Journal of Science, No. iii. p. 1, 

 and No. x. p iiG5. 



f Edinburgh Journal of Science, No. ii. p. 52. 



