(b SUMMARY OF CURRENT RESEARCHES RELATING TO 



upwards light. The light can also reach the crystal K horizontally 

 through a diaphragm : H is the hemisphere. After total reflection 

 ;it the flint-glass-crystal surface the light rays pass into the reflection- 

 prism P, and through the ocular-scale S&, in which lies the limit-line 

 (Grenzlinie) and are observed through the ocular 0, which can be 

 tilted about the hinge A, and is independent of the ocular scale. The 

 ocular tube can be inclined by means of the position-screw St, and the 

 hemisphere can be rotated by the screw-head G. It will be noted that the 

 hemisphere partly functions as a telescope objective. The angle really 

 measured is that between the normal to the crystal face and the limit- 



Fig. 9. 



rays yielding the limit-line, i.e. the limit-angle of total reflection. The 

 pillar of the instrument is provided with a bayonet-clutch by which it 

 can be secured to a cast-iron base for laboratory use. In the portable 

 form the same clutch is attached to an arrangement on the floor of 

 the case. A diaphragm cap fits on to the ring R, so that extraneous 

 light can be excluded and the incident beam admitted through a little 

 window. Ordinary daylight suffices for the illumination, and the accu- 

 racy attainable is within one to two units of the third place of decimals. 

 The readings are taken on the cylindrical-shaped ocular scale S# and 

 range up to n = 1 -85. The axis of this cylinder is such that, on being 

 tilted into the vertical position, it would, if the prism were removed, 



