MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 387 



two colours had mingled before their arrival at a single eye, the mind will perceire 

 the two colours distinctly one or the other alternately predominating either partially 

 or wholly over the disc. In the same manner the mind perceives no trace of violet 

 when red is presented to one eye and blue to the other, nor any vestige of orange 

 when red and yellow are separately presented in a similar manner. These experi- 

 ments may be conveniently repeated by placing the coloured discs in the stereoscope, 

 but they have been most usually made by looking at a white object through dif- 

 ferently coloured glasses, one applied to each eye. 



In some authors we find it stated, contrary to fact, that if similar objects of dif- 

 ferent colour be presented one to each eye, the appearance will be that compounded 

 of the two colours. Dr. Reid* and Janin are among the writers who have fallen 

 into this inconsiderate error, which arose no doubt from their deciding according to 

 previous notions, instead of ascertaining by experiment what actually does happen. 



^ 15. 



No question relating to vision has been so much debated as the cause of the single 

 appearance of objects seen by both eyes. I shall in the present section give a slight 

 review of the various theories which have been advanced by philosophers to account 

 for this phenomenon, in order that the remarks I have to make in the succeeding 

 section may be properly understood. 



The law of visible direction for monocular vision has been variously stated by dif- 

 ferent optical writers. Some have maintained with Drs. Reid and Porterfield, that 

 every external point is seen in the direction of a line passing from its picture on the 

 retina through the centre of the eye ; while others have supposed with Dr. Smith 

 that the visible direction of an object coincides with the visual ray, or the principal 

 ray of the pencil which flows from it to the eye. D'Alembert, furnished with im- 

 perfect data respecting the refractive densities of the humours of the eye, calculated 

 that the appaVent magnitudes of objects would differ widely on the two suppositions, 

 and concluded that the visible point of an object was not seen in either of these di- 

 rections, but sensibly in the direction of a line joining the point itself and its image 

 on the retina; but he acknowledged that he could assign no reason for this law. Sir 

 David Brewster, provided with more accurate data, has shown that these three lines 

 so nearly coincide with each other, that " at an inclination of 30°, a line perpendicular 

 to the point of impression on the retina passes through the common centre, and does 

 not deviate from the real line of visible direction more than half a degree, a quantity 

 too small to interfere with the purposes of vision." We may, therefore, assume in all 

 our future reasonings the truth of the following definition given by this eminent phi- 

 losopher: — "As the interior eye-ball is as nearly as possible a perfect sphere, lines 

 perpendicular to the surface of the retina must all pass through one single point, 



* Enquiry, Sect. xiii. 

 3 D 2 



