Bearing in mind the want of definition beyond 3°, and the indistinctness caused by a diseased central area, it will not be surprising to find that form vision in these cases is imperfect throughout, though the colour perception outside such area may be unimpaired. But, practically, men suffering from this disease are colour blind to coloured objects, such as a signal light on a railway or a ship’s light at sea. They may see that there is light at the distant signal or on the bow of a vessel, but will be unable to interpret correctly the colour. The colours which fail to make visual impressions are the reds and greens. Some will distinguish yellow, and very nearly all will distinguish blue with the centre of the eye. If a bright spectrum be thrown on the screen, and a tobacco-blind person be requested to name the colours of the different parts pointed out to him, it is often the case that as his eyes follow the pointer he will tell you that in the extreme red he sees no light, but in the bright red he sees dull white. The bright yellow he will tell you is a pale yellow or white, according as his case is a moderate or bad one; the green he will call white, and the blue and violet he will designate correctly. At the same time that his eye is turned away to another colour, he will see the true colour of the part of the spectrum which he has just incorrectly named, but it will disappear again as he turns his eyes back again. This tells us that his sense of colour is apparently unaffected outside the diseased area. [Illustration: FIG. 35. _Left Eye._ _Right Eye._]