BLUE GLAss. The question whether blue or white glass is the best for the portrait-room, has been frequently discussed. Blue glass increases the time of exposiut more or less, according to the depth and shade of colour, but not to such an extent as to constitute a practical objection to its use. On the other hand, it acts beneficially in modifying the glare of light which is sometimes injudiciously admitted into the glass room, and in this way relieves the eye of the sitter from a painful annoyance, which causes an unpleasent ex pression. Blue glass stained with cobalt is said to be very perma nent in colour. White glass containing manganese is acted on by light, and becomes of a reddish tint, which is very objectionable. When the glass room has a good aspect, and collodion positives only are required to be taken in it, glass slightly tinted with blue may be very advantageously employed.
BLuz MENISCUS. It has been proposed to use a blue meniscus instead of an achromatic lens for taking views, partly on the ground of economy, and partly bec,ause it was believed that a better picture might be produced. But these supposed advantages of a blue lens
have not yet been demonstrated practically ; on the contrary, a blue lens appears to have the disadvantage of increasing the time of expostut, and of yielding an inferior picture to that produced by an achromatic lens. This might have been expected; for an achro matic lens not only combines two of the coloured foci, but com presses together, so to speak, all the other coloured foci, some of which are more or less actinic ; while a blue lens merely stops certain coloured rays from passing through it, but leaves the foci of those which do pass through very widely separated. It seems impossible to obtain a satisfactory picture with either a colourless or blue meniscus, in consequence of their having no single actinic focus in which the principal actinic rays are collected. Besides this objec tion, no single lens can have its spherical aberration diminished or corrected when its focal length and the radius of its anterior surface are given quantities ; as happens in the case of the landscape lens.