ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



363 



has arranged a diaphragm with two rectangular openings, of 3 by 5 mm. 

 cross section, at a distance apart of 1 mm. Above this diaphragm is 

 a calcspar rhomb 24 mm. high, whose plane-parallel faces make an 

 angle of 55° with the optic axis. The transmitted rays of light under- 

 go double refraction, and therefore form four images in the Microscope. 

 Since the two slits are near together, both the central images partly 

 overlap. In the real imaged made by the telescope objective this posi- 

 tion is stopped out, so that the rest of the light is screened off. In this 

 position also the image of the one slit due to the ordinary ray coincides 

 with the image of the other slit clue to the extraordinary ray ; and the 

 illumination then consists of equal parts of polarised light, if the two 

 openings have received equal illumination. If an absorbing substance 

 be placed before the one opening, then the intensity of the light falling 



Fig. 75. 



on it naturally becomes weaker than that incident on the other ; and 

 in consequence the portion of light polarised in the one direction 

 exceeds that polarised in the other. This effect is recognisable by the 

 occurrence of certain interference effects which are wanting in un- 

 polarised light. The light from the other slit is now weakened in the 

 usual manner, either by the insertion of a smoked glass wedge or by 

 the rotation of a nicol, applied under the opening, on a divided circle 

 whose vernier reads to minutes. This weakening is continued until 

 both portions of polarised light again become equal and the interference 

 effects disappear. It is, therefore, clearly on these interference effects 

 that the adaptability of the instrument depends. In homogeneous 

 light this interference brings out with great distinctness the bright and 

 dark bands of a Savart's plate, and for this purpose a small telescope F 

 (fig. 74), adjusted for infinite distance, of sevenfold magnifying power, 

 is used. Between this telescope and the vart's plate S a nicol N t is 



