ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 275 



The instrument, when intended to be used for the purposes here- 

 after mentioned, must be adjusted as shown in fig. 45, which 

 represents a projection upon a horizontal plane of the parts inter- 

 posed between the eye and the mirror. 



s s Direction of the slit. 



pp „ vibration of the polarizer. 



aa „ „ „ analyser. 



00 „ „ „ ordinary ray in the selenite film. 



e e J, „ „ extraordinary ray in the same. 



In this arrangement of the instrument, when sufficiently strong 

 parallel rays are received from the mirror (either bright, diffused 

 daylight, direct sunlight, petroleum- or gaslight), a dark interference- 

 band will be seen in the spectrum in the field at the point corre- 

 sponding to the Fraunhofer line E, which moves from E to F or 

 E to D, more or less according to the tint of the selenite film. 



The resulting intensity of the light proceeding from the analyser 

 (apart from the loss at the surface) is under the above condition for 

 every given colour 



E^ = r2 sin2 -d(y - a), (1) 



in which r^ is the intensity of the incident light, X the wave-length, 

 d the thickness of the selenite film, y the greatest and a the smallest 

 principal refraction quotients of the selenite for the given wave- 

 length. The dark interference-band appears in every part of the 

 spectrum for which the condition 



^ = 2(.-l)^-^^ (2) 



is fulfilled. In this equation n is the ordinal number of the dark 

 interference-band. It has the value 1 for the thickness o of the plate. 

 The colour region red I. Ord., and purple II. Ord. (red I. Ord. of 

 the ordinary selenite films) lies within the value 2, and the region 

 red II. Ord. and purple III. Ord. (red II. Ord. of the ordinary selenite 

 films) lies within the value 3 for n, whilst the value of X for these 

 regions lies between the 490 and 545 millionth-millimetre of 

 Angstrom's scale ; F Fraunhofer coinciding with 486, E with 527, 

 and D with 589. The interference-band of the red II. Ord. is more 

 sharply limited than that of the red I. Ord. 



The object on an ordinary slide is now brought into the field of 

 view (showing the spectrum with the dark interference-band) and is 

 moved till it lies over this band. If the object is singly refracting it 

 remains, in all azimuths, dark upon a dark ground. If it is doubly 

 refracting, then it acts as a thickening of the selenite film when the 

 vibration-direction of the ordinary (or extraordinary) ray in the object 

 corresponds with that of the ordinary (or extraordinary) ray in the 

 film. In the contrary case it acts as a thinning of the film. In both 

 cases the doubly refracting objects are illuminated on a dark ground 

 in the spectral colour extinguished by the interference-bands. 



If, however, the spectrum is moved under the object by the 



T 2 



