344 Mr. T. Smith's Investigatio7i of certain PhacnomeJia of 



before or behind the true focal point. Now, as in the inves- 

 tigation of new phaenomena, with the principles of which we 

 are, as in the present case, utterly unacquainted, every com- 

 bination of the circumstances ought, as far as possible, to be 

 tried till we arrive at the most simple which is capable of pro- 

 ducing the results ; so it appeared proper to try if any one 

 kind of light, or any one position of it, was more efficacious 

 than another in exciting the appearances. The following ex- 

 periment, repeated often with the utmost care, convinced me 

 that an imperfect image of the bright object was required to fall 

 on the retina in order to produce the phaenomena. 



Exp. 8. I placed a strong bright light at the nearest distance 

 to which my vision could adapt itself, and directing both eyes 

 to it, I caused a screen to be interposed between one of them 

 and the light ; so that one of my eyes only was exposed to the 

 bright light, and the image of it was formed perfecdy distinct 

 on the retina. A slip of xdiite paper, illuminated from behind 

 me, was held so near my eyes as to appear double; the result 

 was very i-emarkable. Of the two images, that which was seen 

 by the exposed eye appeared darker than that which was seen 

 by the shaded eye ; but both appeared distinctli/ white, without 

 any tinge whatever of green or red. In performing this expe- 

 riment, great caution is required that the exposed eye be 

 adapted correctly to the distinct vision of the flame; for by 

 much observation I have found that a small error in this re- 

 spect, such as occurs when the eye becomes dazzled, is suffi- 

 cient 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 v/hite paper. 



The difference of brightness observed in the two images in 

 this experiment is undoubtedly owing to the operation of the 

 affection of sight, mentioned in Note "*, p. 255, &c. as may be 

 pi'oved by shading or exposing both eyes, by turns. When 

 the images appear unequally bright, by shading both eyes, the 

 darker image acquires the same luminousness as the brighter 

 one ; and by exposing both, the bright image becomes of the 

 same shade as the dark one. 



Having thus ascertained that bright light failed in eliciting 

 the phaenomena when it formed a distinct image on the retina, 

 it remained to try the effects of different kinds of light. The 

 results of numerous observations carefully made with the diffe- 

 rent primary colours, are shown in the following experiment. 



Exp. 9. I raised a broad yellow flame in the manner recom- 

 mended by Sir David Brewster for the construction of a mo- 

 nochromatic lamp : this I placed in the position F, fig. 1, near 

 my right eye, and applied a tube, blackened within, to my 



M 



