Any kind of print may be coloured in oils— bromide paper being the most widely used— but owing to the oily nature of the colours it is necessary to size the print first in order to prevent the colours sinking into the paper. To make a suitable size, dissolve one pennyworth of clear patent size (obtainable at an oil-shop) in one pint of hot water, and when nearly cool give the picture a coat, and set aside to dry in a warm place. The coat of size must not be too thick, or it may peel off at a later stage and bring the colours with it. The brushes should be varied both in kind and size, according to the work. There should be provided stout hog-hair brushes, some thin badger brushes, and a few small sables ; a badger softener is also useful. Megilp is used for thinning the colours, and a plentiful supply of turpentine should be provided for washing the brushes.
The methods of applying the actual colours vary considerably. It is usual to apply a suit able tint to the deepest shadows, and to work from this to the highest lights, using a more solid colour as the work proceeds, the reverse of water-colour painting. Drapery and costumes in portrait work may first be covered with transparent colour, working into it the various tints for the shadows, half-tones, and high lights. After the first tints have been put in, it is usual to let the canvas dry and to rub over with poppy oil, removing the excess with chamois leather ; the more delicate colouring is then worked in. It is difficult to give precise in structions for colouring, and the worker must to a large extent be guided by his own tastes. When the painting is completed it should stand on one side for a month before varnishing with copal or mastic, and in no case must the varnish be applied until the painting is dry. Copal is a hard and durable varnish, but mastic is widely used because it dries quicker and, if necessary, may be removed easily.
For lantern slides transparent oil colours must be used. They are sometimes put on with a brush in the same way as water colours, but it is more general to dab on the colouring with the finger-tip. The best way of obliterating the
grain of the finger-tip is to work upon the slide placed at different angles so as to cross the markings and break them up very lightly into a series of small dots. The finer details are best coloured by means of a fine sable brush, but too much colour must not be put on, as it is better to under-paint than over-paint, it being easier to add the colour than it is to take it away. A useful dodge is to put the oil colours at first on the plain glass side, from which they may easily be removed if any error is made, or the colouring may be put on the cover glass of the slide. Parts of the slide may be coloured with dyes and parts with oil and water colours with good effect, but, no matter what process of colouring is adopted, some practice is neces sary, particularly with oils, which are the most difficult for a beginner to use satisfactorily. A slide for colouring in oil or water colours must be lighter and brighter than those for ordinary use, and it is important that they be dried in a room free from dust, as any specks or hairs upon the slide will show up very prominently when magnified upon the screen. P. R. S.
COMA (Fr., Aberration zonale ; Ger., Zonen abirrung, Nebeliger sawn) Synonyms: oblique spherical aberration, zonal aberration. A defect resulting from the unequal magnification of the different zones of a lens, these zones being defined as imaginary circles dividing the surface of the lens into concentric rings. As a result of coma the image of a bright point of light towards the margin of the field of view, produced by oblique rays, will be rendered as a comma- or pear-shaped blur—whence the name " Coma." Coma may occur in a lens other wise well corrected for chromatic and spherical aberration, and is approximately removed by careful design of the curves of the objective and precise selection of the different kinds of glass.