OPTICS. 



pression may be conveyed from the one 

 eye to the other. In order to ascertain 

 it" this was the case, \ve placed a blue 

 glass in front of the unexcited eye that 

 gave the red colour, and the effect of 

 this was to convert the green image 

 seen by the other eye into a greenish 

 sulphur yellow colour. In this case, 

 the right eye must have had it s image 

 modified by the image in the left eye. 

 L in the preceding experiment we sub- 

 stitute a candle in place of .the slip of 

 paper, the green image of the candle 

 seen by the excited eye, will appear to be 

 surrounded with a bright blue margin 



16. Insensibility of certain eyes to 

 particular colours. Various cases have 

 been described, in which persons capa- 

 ble of performing the most delicate 

 functions or vision, are unable to distin- 

 guish particular colours, and, what is 

 certainly a remarkable fact, this imper- 

 feclion runs in families. Mr. Huddart 

 mentions in the Phil. Trans, for 1777, 

 the case of one Harris, a shoemaker at 

 Allonby, in Cumberland, who could 

 only distinguish black and white. He 

 was unable, when a child, to distinguish 

 the cherries on a tree from the leaves, 

 by any other means than their shape 

 and size ; and was surpiised to find that 

 his companions could discern them at a 

 much greater distance than he could, 

 although he saw objects in general as 

 well as they did. He had two brothers, 

 almost equally defective, one of whom 

 constantly mistook orange for grass 

 green, and light green for yellow. He had 

 two other brothers and sisters who, as 

 well as their parents, had no such defect. 



Another case of a Mr. Scott is de- 

 scribed by himself in the Phil. Trans. 

 for 1778. He did not know any green 

 colour : a pin c colour and a pale blue 

 were perfectly alike to him. A full red 

 and a full green were so alike, that he 

 often thought them a good match ; but 

 yellows, light, dark, and middle, and all 

 degrees of blue, except pale sky blue, he 

 knew perfectly well, and he could discern, 

 with particular niceness, a deficiency 

 in any of them : a full purple and a 

 deep blue, however, sometimes baffled 

 him. Mr. Scott's father, his maternal 

 uncle, and one of his sisters, and her 

 two sons, had all the same detect. 



Our illustrious countrymen, Mr. Du- 

 gald Stewart, Mr. Daiton, and Mr. 

 Troughton, experience the same inabil- 

 ity to distinguish certain colours. Mr. 

 Stewart, we believe, first perceived this 

 defect vhen one of his family was 



calling his attention to the beauty of 

 the truit of the Siberian crab, which he 

 could not distinguish from the leaves, 

 but by its form and size. Mr. Daiton 

 cannot distinguish blue from pink by 

 daylight ; and in the solar spectrum the 

 red is scarcely visible, the rest of it 

 appearing to consist of two colours, 



S:llovv and blue. Mr. R. Tucker, son of 

 r. Tucker, of Ashburton, mistakes 

 orange for green, like one of the Har- 

 rises. He cannot distinguish blue from 

 pink, but always knows yellow. He 

 describes the colours of the spectrum as 

 follows : 



1. Red, mistaken for Brown. 



2. Orange Green. 



3. Yellow generally known, but 

 sometimes taken for Orange. 



4. Green, mistaken for O.ange. 



5. Blue Pink. 



6. Indigo Purple. 



T. Violet Purple. 



Mr. Harvey has described in the 

 Edinburgh Transactions the case or a 

 tailor, now alive, and aged sixty, who 

 could distinguish with certainty only 

 white, yellow, and grey. On one occa- 

 sion he repaired an article of dress 

 with crimson, in place of black silk ; 

 and on another occasion he patched the 

 elbow of a blue coat with a piece of 

 crimson cloth. He regarded Indigo and 

 Prussian blue as black ; he considered 

 purple as a modification of blue ; and 

 green puzzled him extremely. The dark- 

 er kinds he considered to be brown, and 

 the lighter kinds as pale orange. He ex- 

 perienced no difficulties with good yel- 

 lows. His notions of orange were im- 

 perfect. The reddish oranges he termed 

 brown, and the lighter kinds yellow. 

 He considered carmine, lake, and crim- 

 son to be blue. The solar spectrum he 

 regarded as consisting only of yellow 

 and li^ht blue. None of the family of 

 this person had the same defect. 



Dr. Nicol has recorded a case in 

 the Medico -C hi rur ^ ical Transactions, 

 where a person who was in the navy, 

 purchased a blue uniform coat and 

 waistcoat, with red breeches to match 

 the blue ; and he has mentioned a second 

 case, in which the defect was derived 

 through the father ; and a third, in which 

 it descended through the mother. 



In the case of a gentleman in the 

 prime of life, on whom we have our- 

 selves made experiments, only two 

 colours were perceived in the spectrum 

 of four colours, in which there was 

 only red, green, blue, and violet. The 



