A test should be one which can be carried out as rapidly as is possible with absolute efficiency; of two equally efficient tests the one which takes the least time must be selected. A test, therefore, should have no unnecessary details which though of theoretical interest are not concerned with the object in hand. The test should be made as easy and as little complicated for the examiner as possible. III. _Persons to be excluded._--We wish to exclude all those individuals who are included in the following three classes: (1) Those who see three or less colours in the spectrum. (2) Those who, whilst being able to perceive a greater number of colours than three, have the red end of the spectrum shortened to a degree incompatible with their recognition of a red light at a distance of two miles. (3) Those who are unable to distinguish between the red, green, and white lights at the normal distance through defect or insensitiveness of the cerebro-retinal apparatus when the image on the retina is diminished in size. I will now explain why these three classes of persons should be excluded. The first class includes the trichromic, the dichromic, and the totally colour-blind, in accordance with the facts previously stated. The trichromic never, in ordinary circumstances, mistake green for red, but confuse yellow with green or red. Colour is a feeble quality of objects to them, and nervousness or excitement may reduce them to the condition of the dichromic. The dichromic are liable to mistake a green light for red, and vice versa. It is very important that persons belonging to the second class should be excluded, and yet none of the ordinarily used tests detect them. The rays of red at the extreme left of the spectrum are the most penetrating, as may be seen by looking at a light or the sun on a foggy day, or through several thicknesses of neutral glass. It is chiefly by these rays that we recognise a red light at a distance; and it is therefore of great importance that a sailor or engine-driver should be able to perceive them. The third class contains persons who are able to distinguish colours easily when they are close to, but fail to distinguish them at a distance, owing to the nerve-fibres supplying the central portion of the retina being impaired. As a light at a distance occupies the central portion of the visual field, it is essential that the corresponding portion of the retina should be normal. There are cases of central scotoma for colours with perfect form-vision; these would, therefore, not be detected by a test for visual acuity. This class also includes those who without having a scotoma are unable to distinguish between colours at the normal distance when the image on the retina is diminished in size.