196 



DISCOVERY 



the painter. If, tlicrcforc, a motliod could be devised 

 by which the person viewing the picture could sec it 

 witlvmt hi-^ view being interfered with by this surface- 



KOR PROJECTING POLARISED 

 OLD PICTURES. 



effect, it would enable the person"to see the picture as 

 it was when it was first painted or as it would be if it 

 were properly restored. By means of his polarised- 

 light apparatus M. Lambert has made this possible. 



A word now on polarised light. Ordinary hght, such 

 as sunlight or electric light, consists of transverse vibra- 

 tions, the vibrations taking place in all directions at 

 right angles to the direction of the rays. There are 

 certain bodies, however, such that when light is passed 

 through them the transverse vibrations all take place 

 parallel to one definite direction only. The ray of light 

 is then said to be plane-polarised. 



The usual means of obtaining plane-polarised light 

 is by means of a prism called a Nicol's-prism, which is 

 made from a crystal of Iceland Spar (calcium car- 

 bonate). A Nicol's-prism may be used not only for 

 producing polarised light (when it is called a polariser), 

 but also for determining the plane in w^hich the light is 

 polarised, when it is said to be used as_an analyser. 



Let us consider the Nicol's-prism as an analyser. 

 If the light falling on the Nicol be nnpolarised, the 

 intensity of the light which will get through will remain 

 the same when the prism is rotated round the light ray 

 as axis. If the light be polarised, however, the 

 intensity of the light varies from a maximum to zero 

 as the analyser is rotated. It is not hard to understand, 

 therefore, that if a mixture of polarised and unpolarised 

 light fall on the analysing prism, it is not a matter ot 

 difficulty to rotate it so that the unpolarised light 

 alone gets through — i.e. if the eye looks through the 

 prism, only the unpolarised lighf is seen by the ob- 

 server. When the analysing Nicol is so rotated that 

 none of the polarised light gets through, it is said to 

 be in the position of extinction. But, of course, even 

 in the position of extinction the unpolarised light 



passes through the Nicol's-prism and is seen by the 

 observer. It is these facts which are made use of by 

 M. Lambert. 



His method is to light up the picture under examina- 

 tion by polarised light and then to examine it through a 

 Nicol's-prism. Under these conditions the light re- 

 flected by the surface, being for the most part polarised, 

 is extinguished by the Nicol's-prism when it is held in 

 the position of extinction, whilst the light passing: 

 through the varnish is depolarised by diffusion on th^ 

 surface of the matter composing the colours and, beiny 

 unpolarised. reaches the eye through the prism. In 

 this way an observer sees the painting itself. 



The apparatus consists of a source of intense light, 

 an arc or incandescent lamp with a low voltage. This 

 light is enclosed in a lantern, provided with a con- 

 denser, followed by a lens destined to make the rays 

 parallel during their passage through the polarising 

 apparatus (a Nicol's-prism). A diverging lens then 

 enlarges the pencil of luminous rays and lights up the 

 entire picture, the plane of which is almost normal to 

 the axis. 



" Although this position is very disadvantageous 

 under ordinary conditions," the inventor has explained. 

 " the observer, looking through the Nicol's-prism, is 

 able by slightly turning the instrument in his hand to 

 find a position in w'hich the superficial reflections are 

 completel}' suppressed. In this way, as you will see, 

 an old and dull-looking picture becomes perfectly 

 distinct and full of vigour ; its surface appears to have 

 been restored ; colours become more intense, and de- 

 tails which do not attract the attention seem to assume 

 the value they had when the work was painted." 



The first picture to be submitted to the action of 



Fig. :.— POSITION OH THE OBSERVER WHEN LOOKING .\T AN 

 OLD PICTURE THROUGH A NICOL'S-PRISM. 



polarised light and then viewed through the Nicol's- 

 prism by one member of the Academy after the other 

 was an old portrait of Gabrielle d'Estrees, a work con- 



