514 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the liquid, the other is in close connection with the observation-tube. 

 Both telescope and glass cylinder are rotatory round an axis lying in 

 the plane of the front face of the cylinder. The floor-plate of the trough 



Fig. 131. ] 



is illuminated with a streak of monochromatic light. If the telescope 

 is so placed that the cross-threads coincide with the limiting line of 

 total reflection, the prism angle is then equal to the angle of total 



