The light of the mercury vapour lamp is different from that of all other lamps. Its peculiarity is that it contains practically no red rays, most of the light being yellow, with a certain proportion of green and blue. The result is a light of a peacock-blue colour. The absence of red rays alters colour-values greatly, scarlet objects appearing black; and on this account it is impossible to match colours by this light. In many respects, however, the deficiency in red rays is a great positive advantage. Every one who has worked by mercury vapour light must have noticed that it enables very fine details to be seen with remarkable distinctness. This property is due to an interesting fact. Daylight and ordinary artificial light is a compound or mixture of rays of different colours. It is a well-known optical fact that a simple lens is unable to bring all these rays to the same focus; so that if we sharply focus an image by red light, it is out of focus or blurred by blue light. This defect of the lens is called “chromatic aberration.” The eye too suffers from chromatic aberration, so that it cannot focus sharply all the different rays at the same time. The violet rays are brought to a focus considerably in front of the red rays, and the green and the yellow rays come in between the two. The eye therefore automatically and unconsciously effects a compromise, and focuses for the greenish-yellow rays. The mercury vapour light consists very largely of these rays, and consequently it enables the image to be focused with greater sharpness; or, in other words, it increases the acuteness of vision. Experiments carried out by Dr. Louis Bell and Dr. C. H. Williams demonstrated this increase in visual sharpness very conclusively. Type, all of exactly the same size, was examined by mercury vapour light, and by the light from an electric incandescent lamp with tungsten filament. The feeling of sharper definition produced by the mercury vapour light was so strong that many observers were certain that the type was larger, and they were convinced that it was exactly the same only after careful personal examination. Mercury vapour light apparently imposes less strain upon the eyes than ordinary artificial light, and this desirable feature is the result of the absence of the red rays, which, besides having little effect in producing vision, are tiring to the eyes on account of their heating action. The light is very highly actinic, and for this reason it is largely used for studio and other interior photographic work. In cases where true daylight colour effects are necessary, a special fluorescent reflector is used with the lamp. By transforming the frequency of the light waves, this reflector supplies the missing red and orange rays, the result being a light giving normal colour effects.