of Vision described by Mr. Smith of Fochabers, 173 



by an increase of intensity. If at the side of the eye we hold 

 the light within two inches of the retina, and in the front of it 

 within six inches, then the degrees of illumination are as 4 to 

 36 ; so that a light nine times greater should be used in front 

 of the eve to produce, ceteris paribus, the same effect. 



In repeating Mr. Smith's experiment No. 8, I do not find 

 that the two images are colourless. The green of the one is 

 comparatively faint, but the yellowish red colour of the other 

 is distinctly visible, though it also is less decided than before, 

 owing to the causes which I have explained above. In order, 

 however, to prove that this diminution of effect is not owing 

 to the distinctness of vision, let the experiment be made ex- 

 actly as described by Mr. Smith, and let the eyes be adjusted 

 to vision more remote than the exciting light, every thing else 

 remaining the same ; the tints of the two slips will remain un- 

 changed; whereas, according to Mr. Smith, the slips ought in- 

 stantly to appear red and green, as in the original experiment. 



Mr. Smith next proceeds to show in his 9th experiment, 

 that homogeneous coloured light will not excite the eye to see 

 the red and green colours in question ; and that in this case 

 the image in the unexcited eye is white instead of red ; while 

 that in the excited eye has a colour complementary to the ex- 

 citing light. This result is exactly conformable to what might 

 have been expected, the phaenomena exhibited by the excited 

 eye being those of accidental colours. I have found, however, 

 that the image seen by the unexcited eye is not always white. 

 When the homogeneous light is red, the colour of the image 

 seen by the unexcited eye is fully as red as it is when the exci- 

 ting light is white. The reason of this is, that as the comple- 

 mentary colour of the homogeneous red is green, the natural 

 reddish colour of candle-light is heightened by contrast. 

 When other homogeneous colours are used as exciting lights, 

 the colour of the image in the unexcited eye varies as might 

 be expected, but is always less decided than in the case of red 

 light. It is proper here to observe, that Mr. Smith's experi- 

 ments were made with coloured papers; but such colours are 

 surely far from homogeneous, and therefore it is not safe to 

 deduce inferences respecting a peculiar action of homogeneous 

 light from experiments in which homogeneous light was not 

 employed. Mr. Smith, indeed, mentions that he used the 

 yellow flame of the monochromatic lamp ; but even if he suc- 

 ceeded in cutting off the red and blue rays with which that 

 flame is generally accompanied, — it is a colour of so peculiar 

 a character, and of so little brilliancy, that no satisfactory 

 result could be obtained with it. 



These observations lead to practical results of some utility. 



