S8 dove's description of an Apparatus 



Under enters into I, but the other, which is on the narrower cylinder, 

 pEisses through, so that on the side toward k a second crystal is screwed 

 in, whose axis may in this manner be made to assume at pleasure any 

 angle to the axis of the first crystal. 



The ring m, nearly in the focal distance of k, is intended for the re- 

 ception of cooled glasses, thin laminae of gypsum, and amethysts. Fast- 

 ened to a pin, its central point is exactly in the axis of the instrument, 

 when the pin is exactly vertical. Similar rings of wood, provided with 

 straight pins, may be placed in the case of the stand s^. Biaxal crystals 

 are fastened to the pins, so that when the ring is turned round the pin, 

 the systems of rings of the two axes pass one after another through the 

 field of view ; if therefore the indexes of the two Nicol's prisms stand 

 at 0° and 90°, the black tufts of the systems of rings lie in a horizontal 

 line. The ring m may also serve for the reception of a micrometric ar- 

 rangement for the systems of rings of the crystals observed in /. 



In order to change the rectilinear into circular polarization, the arms 

 /and g, which revolve round the pegs n and o, contain laminae of biaxal 

 mica* of such a thickness as to produce a difference of path of exactly a 

 quarter-undulation between the two rays, when the axes of those armsff 

 and ^<7 (Plate II. fig. 2.) form with the plane of primitive polarization ee 

 angles of 45° and 135°. Instead of the laminae of mica cooled or com- 

 pressed glasses may be employed, and combined (fig. 5.) in the manner 

 particularly described in the foregoing paper. 



If the two thin plates are turned aside, the rectilinearly polarized light 

 is rectilinearly analysed. In order to analyse circularly, the rectilinearly 

 polarized light/ is brought forwards. In order to analyse rectilinearly 

 the circularly polarized light, / is to be turned aside, and g placed for- 

 ward. The two plates must be brought forward, as in fig. 1 ., when the 

 circular polarized light is to be circularly analysed. The axis of the 

 tliin mica plate is indicated upon its frame. If that axis, instead of 

 corresponding with the points 45° and 135°, passes through other 

 points of graduation, we obtain the phsenomena of elliptic polarization. 

 If a small pin be fixed in the direction of the axis gg, the position of 

 the axis of the lamina of mica may easily be drawn upon the graduation 

 of the stand «.,. 



In order to perform the simple experiments of intensity, it is advan- 

 tageous to uncover the field of view. This is accomplished by a hollow 

 cylinder one inch in height screwed into the somewhat projecting end 

 of the frame of the lens k up towards vi. The aperture of the opake 

 diaphragm in the bottom of this cylinder is 11 line. This well-defined 

 bright circle furnishes a very good object for these experiments. If 



* Although the same phsenomena may be obtained by the determinate incli- 

 nation of a thin plate of uniaxal mica, yet the employment of the biaxal mica 

 appears tome much more convenient. 



