Marine Biology (2+1)

Camera lucida drawings

The specimen must be accurately drawn in terms of proportions and magnifications. A camera lucida is invariably used to ensure accurate drawings of specimens.

A small right- angled and reflecting prism is mounted above the eye-piece of a microscope . The reflecting surface of the prism is silvered except for one small patch in the centre. The prism-mount carries a horizontal arm with an inclinable mirror attached to the end. The mirror should be set at an angle of 45 degree to the  vertical over the drawing paper (though small variations are permissible in the angle of the mirror, too much deviation results in distortion of the drawing).

Light from the drawing paper is reflected by the mirror onto the reflecting face of the prism, which reflects it into the eye of the observer . Light from the microscope eye-piece passes through the small unsilvered patch on the reflecting surface of the prism and enters the eye of the observer along with the light for the paper . As a result, the observer sees the object apparently superimposed on the paper . The observer concomitantly sees the pencil point if a pencil is used for the drawing .

Making a camera lucida drawing

The camera lucida should be attached to the microscope and properly positioned before undertaking the drawing of an organism mounted on a slide and placed on the stage of the microscope. The drawing paper should be kept flat on the right-hand side of the microscope. Invariably, a fairly hard-leaded pencil should be used to obtain precision lines. While drawing with the right hand, the left eye should be at the eye-piece leaving the ' right eye free to look at what is being drawn without raising the head. By this method, only the main outline of the object is traced, which serves as the framework into which other details can be drawn later under the microscope but without the camera lucida.

Effective use of the camera lucida lies in the exact balancing of the intensity of light on the drawing paper and the object respectively. For example, if the light from the object is too bright, the pencil point cannot be seen; on the other hand, if the paper is too bright, the object disappears altogether from view. If the light to the specimen is too intense, i t can be reduced by closing the iris diaphragm of the sub-stage condenser.

Depicting size of the specimen in drawing

To depict the size of a specimen, the stage micrometer is placed on the stage of the microscope immediately after removing the specimen slide but without altering the microscope setup. Subsequently, a space equivalent to 100 or 200 µm of the millimeter scale is drawn alongside the object.