938. Examination of Negatives. X-ray nega tives are examined by placing them on the diffusing front of a strongly-illuminated light box called a negatoscope. Most of these instru ments are arranged to receive the negative directly it is taken from the washing water, the negative being gripped by clips along its upper edge. Care should be taken to arrange the negative the right way round ; it should not be left for long, owing to the risk of melting the gelatine by the prolonged heating.
939. Printing of Positives. Although radio logists generally examine the negative image, many cases occur (e.g. communication to a third. party or illustrations of papers or for confer ences) in which positives must be prepared either on paper by contact, or on lantern plates, by reduction.
The difference, which is generally consider able, between the extreme densities of a radio graphic negative makes it very difficult, if not impossible, to obtain a positive copy with good values through the whole range of the gradation. All personal interventions of the operator, such as masking, tricks of printing, and so on, run a great risk of misrepresenting or falsifying the radiograph, which should form an authentic document. Fortunately, the importance of a
radiograph frequently resides in a small range of densities. In a medical radiograph, for example, it is rare that the detailed structure of bones and lesions of the soft tissues are both of them of interest. In such a case the printing can be arranged to give the best rendering of the important part. In exceptional circumstances two separate prints may be made, one to show the dense parts to the best advan tage and the other to show the lighter details.
940. Various Applications. In addition to the applications already mentioned in medicine and for disclosing defects in metal goods, radio graphy has applications in many industries for the control of manufactured products or for studying processes of manufacture. X-rays have been used with success for the examination of old paintings (A. Cheron, 1921), for distinguish ing between natural and cultivated pearls, etc. Finally, brief mention should be made of the applications to radiography of stereoscopic methods,' facilitating the interpretation of results, and aiding appreciably in the localization of foreign bodies ; also of attempts which have been made to obtain motion-picture records by means of X-rays.