The principal disturbing conditions in making observations are _want of colour education_ and _insufficient diffusion_. In the case of the latter, the first evidence is the disturbance of constancy by the penetrating red ray. A partial remedy is to interpose a white diffusion screen, such as tissue paper. _Time of Observation._--This should not exceed five consecutive seconds, as the keenness of perception decreases by time, but varies for different colours. _Angle of Incidence._--Sixty degrees is safe for most solids, but for bright or polished surfaces, such as varnishes, polished metals, etc., the angle must be lessened as the degree of smoothness increases. For very rough surfaces, such as loosely woven stuffs, etc., care must be taken that the lay of the fibre is uniform. _Distance from the Object._--Ten inches has been adopted for general work, but certain visions require more or less as their focus varies from normal. [Illustration: PLATE X COMPARISON CURVES OF HEALTHY AND DISEASED BLOOD. _To face page 80._] [_Lovibond, Colour Theories._ ] _Unabsorbable Colours._--In addition to the daylight colours already dealt with, there are, in direct lights, colours which do not obey the laws of absorption governing those of diffused daylight. The work already done on these unabsorbable rays has only been incidental, where they happened to interfere with the standardization of diffused daylight colours. The sensations excited are red and violet. They blend, producing red-violet mixtures, but in unequal proportions, the red being dominant. The red is developed in intense lights by constant interception of neutral tint absorbents. In the case of a 4-volt incandescent light, the first absorption simply reduces the light intensity without developing colour; the light is colourless up to 14 units. At 16 units the light begins to assume a reddish hue, which rapidly becomes a brilliant intense red by further interceptions of neutral tint absorbents. Violet is developed by constant absorption by blue standards, which grows in intensity by successive additions up to about 120 units. Beyond this point the brilliancy decreases. Preliminary experiments point to this ray as fatal to vegetation, and presumably also to lower forms of organic life. There remains a factor of considerable importance which has not yet received the attention it deserves, the physiological changes resulting from environment. This aspect of the question has come under the notice of the author by measuring the vision of experts who excelled in given hues. It was generally found that their vision was sensitive to a small increment of their particular colour in harmonies where it was silent to a normal vision. In seeking an explanation of this phenomenon there are at least three possible lines for working: First. Is the vision naturally more sensitive to that particular energy? Second. Is it by careful education?