604 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



inclined to the vertical so that oblique illumination of the object may be 

 obtained. By using a cylinder with an interior white glazed surface the 



Fig. 70. 



beam of light is intensified. A sheet-iron screen is also attached to the 

 arm so that the operator's eyes are shielded from the rays. 



(4) Photomicrography. 



Interferential Photography ; Variation of Incidence ; Polarised 

 Light.* — M. Ponsot has studied experimentally the interferences of 

 polarised light reflected from a plane mercury surface, the interferences 

 being produced in the thickness of a transparent layer of gelatino- 

 bromide of silver in contact everywhere with the reflecting surface. He 

 was, in reality, repeating Wiener's experiments with, as a variation, 

 Lippmann's arrangement for interferential photography. His experi- 

 ments were made with (1) non-polarised light ; (2) polarised light ; 

 (3) without the mercury mirror. 



1. Non-polarised Light. — Photographs of the spectrum were taken 

 under normal incidence and under increased incidence, notably at 45° in 

 air, which gives an incidence (i^ of about 28° in the gelatin. To get 

 an incidence of 45° in the gelatin a right-angled isosceles prism of 

 crown-barium (l - 55 for yellow light) was used. The examination in 

 white light, under normal incidence, of the photographs shows that the 

 spectrum colours are displaced towards the violet, and the more so as / t 

 is increased. Under an increasing incidence the colours are displaced 

 towards the red. These results are in agreement with theory, and bear 

 upon the choice of objectives in interferential photography as regards 

 regulating the working distance for getting the maximum effects with 

 the active colours. 



2. Polarised Light. — The results obtained are identical with the 



* Coniptes Rendus, cxlii. (1906) pp. 1506-9. 



