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 the white scale, the plants are washed with a sponge and solution of soft soap as soon as their growth is completed, and again before the buds begin to swell. The brown scale may be got rid of by repeated washings with one of the many insecticides, but it should be applied at a temperature of 90°.

 CAMEO, a term of doubtful origin, applied in the first instance to engraved work executed in relief on hard or precious stones. It is also applied to imitations of such stones in glass, called “pastes,” or on the shells of molluscous animals. A cameo is therefore the converse of an intaglio, which consists of an incised or sunk engraving in the same class of materials. For the history of this branch of art, and for an account of some of its most remarkable examples, see.

The origin of the word is doubtful and has been a matter of copious controversy. The New English Dictionary quotes its use in a Sarum inventory of 1222, “lapis unus cameu” and “magnus camehu.” The word is in current use in the 13th century. Thus Matthew Paris, in his Life of Abbot Leofric of St Albans, in the Abbatum S. Albani Vitae, says: “retentis quibusdam nobilibus lapidibus insculptis, quos camaeos vulgariter appellamus.” In variant forms the word has found its way into most languages, e.g. Latin, camahutus, camahelus, camaynus; Italian, chammeo, chameo; French, camahieu, chemahou, camaut, camaieu. The following may be mentioned among the derivations that have been proposed:—von Hammer: camaut, the hump of a camel; Littré and others: camateum, an assumed Low Latin form from  and  ; Chabouillet and Babelon: , treasures, connecting the word in particular with the dispersion of treasures from Constantinople, in 1204; King: Arabic camea, an amulet.

 CAMERA (a Latin adaptation of Gr. , an arched chamber), in law, a word applied at one time to the English judges’ chambers in Serjeants’ Inn, as distinct from their bench in Westminster Hall. It was afterwards applied to the judges’ private room behind the court, and, hence, in the phrase in camera, to cases heard in private, i.e. in chambers. So far as criminal cases are concerned, the courts have no power to hear them in private, nor have they any power to order adults (men or women) out of court during the hearing. In civil proceedings at common law, it may also be laid down that the public cannot be excluded from the court; in Malan v. Young, 1889, 6 T.L.R. 68, Mr Justice Denman held that he had power to hear the case in camera, but he afterwards stated that there was considerable doubt among the judges as to the power to hear cases in camera, even by consent, and the case was, by consent of the parties, finally proceeded with before the judge as arbitrator. In the court of chancery it is the practice to hear in private cases affecting wards of the court and lunatics, family disputes (by consent), and cases where a public trial would defeat the object of the action (Andrew v. Raeburn, 1874, L.R. 9 Ch. 522). In an action for infringement of a patent for a chemical process the defendant was allowed to state a secret process in camera (Badische Anilin und Soda Fabrik v. Gillman, 1883, 24 Ch. D. 156). The Court of Appeal has decided that it has power to sit in private; in Mellor v. Thompson, 1885, 31 Ch. D. 55, it was stated that a public hearing would defeat the object of the action, and render the respondent’s success in the appeal useless. In matrimonial causes, the divorce court, following the practice of the ecclesiastical courts under the provisions of the Matrimonial Causes Act 1857, s. 22, hears suits for nullity of marriage on physical grounds in camera, but not petitions for dissolution of marriage, which must be heard in open court. It was also decided in Druce v. Druce, 1903, 19 T.L.R. 387, that, in cases for judicial separation the court has jurisdiction to hear the case in camera, where it is satisfied that justice cannot be done by hearing the case in public.

  CAMERA LUCIDA, an optical instrument invented by Dr William Hyde Wollaston for drawing in perspective. Closing one eye and looking vertically downwards with the other through a slip of plain glass, e.g. a microscope cover-glass, held close to the eye and inclined at an angle of 45° to the horizon, one can see the images of objects in front, formed by reflection from the surface of the glass, and at the same time one can also see through the transparent glass. The virtual images of the objects appear projected on the surface of a sheet of paper placed beneath the slip of glass, and their outline can be accurately traced with a pencil. This is the simplest form of the camera lucida. The image (see fig. 1) is, however, inverted and perverted, and it is not very bright owing to the poor reflecting power of unsilvered glass. The brightness of the image is sometimes increased by silvering the glass; and on removing a small portion of the silver the observer can see the image with part of the pupil while he sees the paper through the unsilvered aperture with the remaining part. This form of the instrument is often used in conjunction with the microscope, the mirror being attached to the eye-piece and the tube of the microscope being placed horizontally.

About the beginning of the 19th century Dr Wollaston invented a simple form of the camera lucida which gives bright and erect images. A four-sided prism of glass is constructed having one angle of 90°, the opposite angle of 135°, and the two remaining angles each of 67°. This is represented in cross-section and in position in fig. 2. When the pupil of the eye is held half over the edge of the prism a, one sees the image of the object with one half of the pupil and the paper with the other half. The image is formed by successive total reflection at the surfaces b c and a b. In the first place an inverted image (first image) is formed in the face b c, and then an image of this image is formed in a b, and it is the outline of this second image seen projected on the paper that is traced by the pencil. It is desirable for two reasons that the image should lie in the plane of the paper, and this can be secured by placing a suitable lens between the object and the prism. If the image does not lie in the plane of the paper, it is impossible to see it and the pencil-point clearly at the same time. Moreover, any slight movement of the head will cause the image to appear to move relatively to the paper, and will render it difficult to obtain an accurate drawing.

Before the application of photography, the camera lucida was of considerable importance to draughtsmen. The advantages claimed for it were its cheapness, smallness and portability; that there was no appreciable distortion, and that its field was much larger than that of the camera obscura. It was used largely for copying, for reducing or for enlarging existing drawings. It will readily be understood, for example, that a copy will be half-size if the distance of the object from the instrument is double the distance of the instrument from the copy.

  CAMERA OBSCURA, an optical apparatus consisting of a darkened chamber (for which its name is the Latin rendering) at the top of which is placed a box or lantern containing a convex lens and sloping mirror, or a prism combining the lens and mirror. If we hold a common reading lens (a magnifying lens) in front of a lamp or some other bright object and at some distance from it, and if we hold a sheet of paper vertically at a suitable distance behind the lens, we see depicted on the paper an image of the lamp. This image is inverted and perverted. If now we place a plane mirror (e.g. a lady’s hand glass) behind the lens and inclined at an angle of 45° to the horizon so as to reflect the rays of light vertically downwards, we can produce on a horizontal sheet of paper an unperverted image of the bright object (fig. 1), i.e. the image has the same appearance as the object and is not perverted as when the reflection of a printed page is viewed in a mirror. This is the principle of the