DISCOVERY 



145 



The drying of their pictures, however, presented 

 difficulties, owing to ignorance of the difference between 

 pure oil and siccative linseed oil ; and to remedy this 

 the galipot method was then adopted — that is to say, 

 colours were first of all ground with turpentine and then, 

 when in the form of powder, diluted with a varnish 

 composed of the said galipot (white resin, a sort of 

 turpentine) and spirits of turpentine. This technique 

 changed but little down the Ages, bringing Professor 

 Laurie to the conclusion that as regards the instruments 

 of their art there was practically no change from the 

 days of Leonardo da Vinci to those of Renoir and the 



one to six diameters ; whereupon it became evident 

 that, in the fight against the picture-forger, photo- 

 graphy and its various applications was to play one of 

 the most important of roles. 



To H. Parenty, of Lille, is due the credit for having 

 discovered that by simple photography it was possible 

 to penetrate the undeniable transparency of the layers 

 of paint on pictures, and thus reconstitute, as it were, 

 certain details in the works of Titian, Rembrandt, 

 Rubens, and other Great Masters that are invisible to 

 the naked eye. This at once led other technologists to 

 ask themselves the question, \\'liy not employ X rays ? 



KIG. 4. 



■ VIERGE DE STEI,I,.\- 



Impressionists. However, the pigments used in paint- 

 ing did vary considerably from period to period, and 

 by applying his method to a large number of works of 

 absolutely certain date, the investigator with the 

 microscope was able to draw up a chronological list of 

 pigments of the greatest utility. Microscopic study of 

 the pigments of a given painting enabled one, then, in 

 many cases, to determine its date appro.xiniately and 

 to declare whether any portions had been retouched by 

 restorers. Advancing this excellent method one more 

 step further, Professor Laurie took micro-photographs 

 of minute portions of ancient canvases, enlarged from 



R.\DIOGR.\PH OK THE " VIERGE DE .STEI.L.\ " SHOWING 



D.\>L\GEU PORTION WHICH HAS BEEN RESTORED. 



In the centre a bar of wood prevented complete passatje of the X rays. 



No sooner said than done. And thus, in 1914, the first 

 researches concerning the radiography of pictures were 

 made in Germany by Faber, as related in the Zeitschrifl 

 fur Museumkunde. Dr. Heilbron, of Amsterdam, 

 continued them with most curious results. Finally, 

 between the middle of 1920 and the present date, we 

 find a Parisian specialist. Dr. Andre Cheron, perfecting 

 the method to such a degree that we are tempted to say 

 the days of the picture forger are numbered. 



Dr. Heilbron 's results must first of all be described. 

 As the subject for his investigations he took a 

 " Crucifixion," by Engelbrechtsz, a picture which was 



