17'i Sir 1). Brewster's Examination of the Phenomena 



the slip of paper are colourless. A bright light is held as 

 near the eye as possible, so as to be seen with perfect distinct- 

 ness, and a slip of paper illuminated by a candle held above 

 the head is placed between the exciting light and the eye, and 

 so near the latter as to be seen double. In this experiment, 

 which he has repeated often with the utmost care, the images 

 of the slip of paper are perfectly colourless, the one seen by 

 the exposed eye being only a little darker than the other. 

 " Jn performing this experiment," says Mr. Smith, " great 

 caution is required that the exposed eye be adapted correctly 

 to the distinct vision of the flame; for by much observa- 

 tion I have found that a small] error in this respect, such as 

 occurs when the eye becomes dazzled, is sufficient to excite 

 those changes in the sensibility to red light which have been 

 proved to be the causes of the green and red appearances of 

 the white paper." 



Now, supposing this experiment and its result to be exactly 

 described by Mr. Smith, four observations present themselves 

 to us. 



1. As the exciting bright light must have been about five or 

 six inches from the eye, and was, I presume, that of an Argand 

 lamp or large candle, it must have subtended an angle of 8° or 

 10°, so that a great part of the exciting light must have been 

 seen indistinctly, as the eye sees only with perfect distinctness 

 a small point in the axis of the eye. In order, too, to see this 

 large light with perfect distinctness, Mr. Smith must have 

 fixed his eye upon the margin of it, so that the other margin 

 of the flame must have been exciting the eye by indistinct vi- 

 sion, at the distance of 8° or 10° from the axis of the eye. 



c 2. If the exciting light were a small and highly luminous 

 object, so that the whole of its margin could be seen pretty 

 distinctly, then its image would fall upon the foramen centrale 

 in the retina, where there is no nervous matter to be excited. 



3. When the exciting light, whether large or small, does fall 

 in a distinct image upon the retina round the foramen centrales 

 it acts upon the part of the retina, which being continually 

 exposed to the action of light, is less easily excited, and a part 

 too which can be proved by direct experiment to be less sen- 

 sible to calorific impressions. Hence we have a distinct reason 

 why an exciting light falling on the central part of the retina 

 does not produce the same insensibility to red light which is 

 produced by the same light acting upon a less used portion 

 of the same membrane. 



4. To these reasons we may add a fourth ; namely, that the 

 light in this experiment is necessarily much fainter from its 

 being held at a greater distance ; though this may be balanced 



