Again, the examiner may think that it might be better to have an apparatus which showed two or three lights instead of only one. I will therefore give my reasons for adopting only one. This point was one which occupied my attention for a considerable time, especially in view of the fact--which, as far as I am aware, I was the first to discover--that simultaneous contrast is increased to the colour-blind. I was naturally anxious to turn this fact to account. I found, however, that I gained nothing by increasing the number of lights, and that in many cases it was a source of error. A second or third light could have been easily added to my lantern, but besides unnecessarily complicating the apparatus it would have served no useful purpose. All the results which are obtained with simultaneous contrast are obtained even more effectively with successive contrast. It will be noticed that when lights are seen in ordinary conditions they are conditions of successive contrast, and not of simultaneous contrast. An observer rarely keeps his eyes definitely fixed on one light whilst he names those adjacent to it, but moves his eyes so that the images of the respective lights fall successively on his foveas. When more than one light is employed, all the disadvantages of matching as against naming are introduced. It will be seen that by presenting one light after another we are fulfilling all the necessary conditions, only that the light is moved instead of the eyes, sufficient time being allowed to elapse to enable a normal-sighted person to readily recognise the true colour of the next light without being confused by the after-image of the one he has just seen. Many nervous normal-sighted would name a yellow light seen between two red lights as green, and it does look green to them from ordinary physiological conditions. They look first at one red light, then immediately at the yellow light, then at the second red light, and then again at the yellow light until they feel sure that the centre light is a green light, and say so. I have never met with a normal-sighted person who has miscalled the unmodified light of my lantern, either red or green. Many humble, nervous normal-sighted persons are under the impression that they are colour-blind, and yet would make perfectly efficient officers. Many of these men have been told by their wives or other persons that they are colour-blind, and, believing this, try to see colours which are not visible to them. I have examined many persons of this description, and have noted the ease and accuracy with which they have gone through the tests for colour-blindness when they have been assured by me that they were normal-sighted. On the other hand, it is often very difficult to convince a self-reliant, colour-blind person that he is colour-blind. He is on the look-out for the small differences which he notices between colours, and the fact of having another light for comparison