la- 

 this paper so as to produce a diffused light. When the crystal 

 is in place in the denser fluid, only those rays which have less 

 than a certain obliquity can enter the crystal, the rest are "totally" 

 reflected and enter the telescope, one of whose cross lines is 

 brought to coincidence with the sharp line marking the limit 

 of total reflection, and its position is read on the horizontal 

 circle. The light is then transferred to the opposite side of the 

 instrument and the limit on that side observed in the same way. 

 Half the angle between these two positions is the limiting angle 

 of incidence whose sine is the index of refraction between the 

 crystal and the surrounding fluid ; the latter being known the 

 refractive index in air of the crystal can be obtained at once. 

 The accessory apparatus consists of extra glass holders, a 

 condenser on stand, a thermometer to test the temperature of 

 the enclosing fluid which will affect its refractive index, <&c. 



46. Norremberg's polarizing apparatus. Specially 

 adapted for crystallographic work. Can also be used 

 as a goniometer; 



E. 13, 1887. Made by Steeg and Renter. 



This consists of a vertical fixed bar to the lower part of which 

 is fixed a pair of mirrors each movable about a horizontal axis 

 for polarizing the light. Three tubes slide by means of side 

 sockets on the upright bar, and can be fixed in any given 

 position by screws. The lowest contains a series of converging 

 lenses ; the middle a series of magnifying lenses. Between these 

 two tubes the crystal section is laid. The upper tube contains a 

 nicol and is graduated on its circumference. There is also a 

 separate fitting to act as a goniometer. 



47. Polariscope with lens for parallel light and a 

 large nicol, for measuring the angle between the axes. 



E. 87. 1877. Made by L. Laurent, Paris. 



On the same principle as the Norremberg polariscope. It has a 

 tripod with a movable joint to the upright, so that the instrument 

 can be put at any angle ; it has two mirrors for polarizing, and 

 a graduated stage. 



48. Grroth's universal apparatus for crystallographic 

 optical research. 



E. 37. 1884. Made by E. Fuess, Berlin. 



This apparatus consists of several parts, which can be fitted 

 together in various ways for various purposes. 



First, there is a vertical stand, on the horizontal base of which 

 is placed a rotating mirror ; on the upright bar two carriers slide, 

 each of which may be clamped by a screw. Into the circular 

 apertures of these slides fit two pairs of polarising apparatus. 

 The one pair for parallel light consists of (1) the polarising part, 



