ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



515 



reflection and we get n = — — , where n = refractive index and e the 



sin e 



angle of total reflection. The method is, as it were, a special case of 



prismatic vision, and can be defined as the method of oblique incidence 



and normal emergence. As the refractive index of the less refractive 



medium (air, /x = 1) is known, there is no limit to the applicability of 



the method. In the case of the very highly refractive liquids, the glass 



parts are made of a specially hard and dense kind. 



If A n, A e be small variations in the values of n and e, then the 



degree of accuracy is given by the equation 



A n = — 



Ae 



sin e . tan e 



Volatile liquids and liquids which easily solidify are difficult to deal 

 with. 



A view of the instrument is shown in fig. 131. The rigidly attached 

 sector S carries a scale graduated from 0° to 75° into half degrees. The 



Fig 133. 



Fig. 132. 



telescope F rotates about the horizontal axis, and a vernier N, by means 

 of the lens L, gives readings to 1'. The coarse adjustment of the tele- 

 scope is by hand motion ; and the fine by a drum-headed micrometer 

 screw M graduated to 1'. The latter appliance adapts the instrument 

 for dispersion readings, for which the previous formula suffices. 



The glass body at the base of the telescope is tapered off and cone- 

 shaped ; its under surface is only a strip 5 mm. broad, which is placed 

 accurately in the axis of rotation of the telescope. Thus the solution- 

 trough can be approximated very closely to the strip, and the distance 

 regulated by the milled screw-head D. The lens B serves for the 

 oblique illumination of the under side of the glass trough. The light 

 source is sufficiently far removed to allow of the formation of a flame- 

 image on the screen before the glass plate. Diaphragms can be placed 

 in the telescope to cut off any reflected images. 



A little window a in the eye-piece (fig. 132) allows, by a method of 

 reflections, the normal adjustment of the telescope-axis to the prism- 

 planes ; small adjustments are imparted by the screw J (fig. 131). 



The method of examining solid bodies is clearly shown in fig. 133. 

 The solid must be in the shape of a prism P, and is placed on a plane 

 piece of glass B with parallel sides, the refracting angle of the prism 



