()79 



however, gained ground, that the layers of air, which are condensed 

 on the surface, exert an appreciable influence on the elliptical 

 polarisation at the reflection, so that in the first place this influence 

 has been inore closely examined. 



3. The used uionochronnttor. The determination of the optical 

 constants took place in an entirely analogous way, as has been de- 

 scribed by one of us '). For the investigation a goniometer has been 

 used, the graduated circle of which can be placed vertical. Before 

 the goniometer there is a monochromator of a very simple structure, 

 which was constructed from material, present in the laboratory. The 

 monochromator consists of a collimator with an aperture 1 : 6, a flint- 

 glass prism of Steiniikil with an angle of refraction of 60°, and a 

 second collimatoi-, which will be referred to in futui-e as the colli- 

 mator of the goniometer. The illumination takes place by means of 

 an arc-lamp of 18 Amperes. A lens of 7 dioptrics forms an image 

 of the crater on the slit of the collimator of the monochromator. 

 Hehind the prism a lens of Jl dioptrics forms a spectrum on the 

 collimator of the goniometer. The axes of the two collimators are 

 placed horizontal, the slits and the edge of the refractiiigangleoftheprisiTi 

 vertical. Care has always been taken, that the image of the crater 

 and the spectrum fall on the middle of the collimator slits. A silvered 

 glass mirror is adjusted to the collimator of the goniometer; it can 

 revolve round an horizontal axis and throws a monochromatic, cylin- 

 drical beam of light at the required angle of incidence on the mirror 

 in the middle of the goniometer. The wa\e-length of the incident rays lies 



o 



between 5790 and 5990 Angstrom-units. During the observations the 

 invariability of this colour-sifting of the incident beam of light is repeat- 

 edly examined. The fringes in the Babinet compensator are always 

 uncoloured. Monochromator and goniometer are mounted on a fii-m 

 foundation, erected free from the floor in the room, in which the 

 observations have been made. The three levelling-screvvs of the legs 

 of the goniometer stand each on two thick pieces of india-rubber, 

 in order to prevent as much as possible the influence of vibrations 

 on the liquid mercury surface. 



4. The goniometer. For the adjustment and the centring of the 

 parts of the goniometer we i-efer to the investigation of one of us'). 

 Only a few points are briefly mentioned here. 



') R. SissiNGH, loc. cit. 

 2) R. SlSSINGH, loc. cit. 



