It will be noticed that three of the discs are similar in every respect. In some of my lanterns the two reds are put at the end of the series of colours and numbered Red 1 and Red 2. This makes no important difference, but the arrangement given here is more convenient. It should be noted that Red 1 corresponds to Red B and Red 2 to Red A. If the electric lamp should get broken the projecting apparatus can be removed and an ordinary kerosene lamp placed behind the aperture. [Illustration: Fig. 4.] 2. _Reasons for special construction._--The lantern has been constructed conformably with the requirements and facts of colour-blindness. All the facts I have given have been considered in constructing the lantern. The examiner, on possessing a lantern for the first time, should carefully test himself with it and ascertain how the different lights appear to him with different conditions of general illumination. It is probable that certain improvements may suggest themselves to him, therefore, I think it will be advisable to deal with certain of these points, as it will help the examiner in the use of the lantern. The colours have never been altered, and I certainly should have altered them if I could have improved the lantern by doing so. I have never met a single colour-blind person who has not been readily and easily detected with my lantern, though I have examined many who have passed other lanterns and in some cases a number of other tests for colour-blindness. In most cases one turn of the wheel will be sufficient to make a colour-blind person disclose his defect.