Royal Microscojpical Society. 215 



III. — On Bichromatic Vision. 



By J. W. Stephenson, F.E.A.S., Treas. K.M.S., and Actuary to 



the Equitable Life Assurance Society. 



(Read before the Royal Microscopical Society, April 3, 1872.) 



It is probably well known to every Fellow of the Eoyal Micro- 

 scopical Society that, by the aid of a double-image prism and a film 

 of selenite, two images may be shown in the field of the microscope, 

 the colours of which will be complementary the one to the other, 

 and that when these images overlap, the resulting image will be, as 

 far as the overlapping extends, of white light ; but it is not, I think, 

 so well known that when, by a suitable arrangement, diflerent 

 colours are made to occupy the two fields of a binocular, the 

 resultant is a combination of such colours, and that if these are 

 complementary to one another, the sensation of colour induced in 

 the brain by the retina of one eye, is neutralized by that which 

 reaches it through the instrumentality of the other, and that by 

 the combination of the two the sensation of colour is entirely lost. 



The pm-port of the present communication is to show how this 

 phenomenon may readily be observed. 



On the stage of the microscope before you, which I described at 

 our last meeting, is a selenite film transmitting the green light of 

 the third wave; this, in the normal condition of the instrument, 

 of course fills both fields, but as the object on the present occasion 

 is to produce difierent colours in the two tubes, I have introduced 

 between the analyzing plate and oie of the binocular prisms a film 

 of mica of such a thickness that it will, according to the direction 

 of its axis, accelerate or retard the transmitted ray half an undula- 

 tion. By the use of this film of mica I have raised the original 

 colour in the right-hand tube half a wave, and thus, whilst the 

 original fine green of the third wave is transmitted to the left eye, 

 the right is filled with the bright red higher up in the same scale. 

 These colours are very nearly complementary, and it will be seen, 

 on looking through the instrument with both eyes, that the field is 

 practically white. 



It will further be found that if, after looking at this colour- 

 less circle for a time, either eye be closed, a faint tint will 

 gradually appear in the other eye, but very inferior in intensity to 

 that which was originally experienced m looking down a single 

 tube ; and it will be observed that the diminution in intensity 

 takes place through that eye which has continuously transmitted 

 the same colour; although the quenching of the colour might be 

 presumed to have ceased, when, by dropping the eyehd, the cause of 

 its primary extinction no longer exists, this is not so ; but must, to 

 a certain extent at least, be consequent on the complementary tint 

 having fallen on the retina of the closed eye. 



