476 Intelligence and Miscellaneous Articles, 



suffices to produce close contact at any part of the spectrum the 

 observation of which is desired. 



For the arrangement of the apparatus, the Mcol without the 

 insertion of the prisms was turned so that one image of the slit 

 vanished ; then the prisms were prefixed and rotated until again 

 only one image of the slit was to be seen. Their incidence -plane 

 then coincides with the principal section of the doubly refracting 

 prism ; and this gives at the same time the point from which the 

 rotation of the Mcol is to be reckoned. In front of the lower half 

 of the slit a rectangular glass prism was fixed, which reflected the 

 light of a laterally placed petroleum-flame into the apparatus. On 

 a level with the upper half stood the telescopes of a small spectro- 

 meter provided with a little turntable, by which the light of a 

 second flame was concentrated upon the upper half of the slit. 

 The tube of the collimator, standing next to the flame, had, in 

 place of the slit, a circular aperture ; and the two telescopes, pro- 

 vided with only the objective-lenses, were placed so that the lens 

 of the second threw upon the upper half of the slit of the photo- 

 meter a sharp image of the aperture, which was in the focus of the 

 first. 



For the observation the telescope next the flame was placed at 

 180°, and its objective lens replaced by a Mcol prism arranged so 

 as to cause the disappearance of the lower image of the upper half 

 of the slit. The tube, with the flame in front of it, was then 

 rotated 40°, the glass prism to be investigated fastened lightly with 

 wax upon the table so that it reflected the light of the flame into 

 the apparatus, and inclined until the lower extraordinary image dis- 

 appeared. The incidence-plane was then perpendicular to the 

 principal section ; and as the extra image of the upper half of the 

 slit was compared with the ordinary one of the lower, the observa- 

 tions hold good for light polarized parallel to the plane of incidence. 



For observation under various incidences, the two conterminous 

 spectra were brought to equal brightness by direct light ; then the 

 collimator- tube, with the flame, was rotated double the desired 

 angle of incidence, and the table with the glass prism so placed 

 that the reflected image of the aperture appeared again at the same 

 spot on the photometer as the direct image, indicated by a mark. 

 If a and |8 are the angles made by the polarization-plane of the 

 Mcol (with equal brightness in both cases) with the principal sec- 

 tion of the doubly refracting prism, then the intensity of the 

 reflected light, in parts of the incident, is 



E= 



tan 2 /3 



In order to make myself independent of accidental alterations in 

 the brightness of the two flames, five successive experiments were 

 made each time, in direct and reflected light, for the same angle of 

 incidence ; and in the calculation each three consecutive ones were 

 combined. The whole of the observations are valid for the place 



