164 THE AMERICAN MONTHLY [June, 



tral part, and perforated with a central aperture ; this 

 permits of the sliding movement within the hollow, for 

 centering purposes, of a disc which carries the axis of 

 the instrument. To the lower end of this short axis are 

 attached the movements for adjusting the crystal, and 

 the lower of which carries the crystal. The adjusting- 

 movements are a pair of circular quadrants arranged at 

 right angles to each other and graduated. They are 

 simpler in construction, and lie much closer together 

 tlian those of the best forms of goniometer now in use 

 for ordinary goniometric and spectrometric work, and 

 are therefore particularly suitable for use in connection 

 with the microscope. The upper quadrant is fixed to 

 the axis ; over it a slider is capable of moving, which car- 

 ries a vernier, and below it the lower quadrant, which 

 in turn is fitted with a slider terminating in the holder 

 which carries the crystal cemented by wax. The ver- 

 niers enable readings of five minutes to be obtained, the 

 same degree- of accuracy as in the case of the circle. 



The glass cell containing the immersion liquid is sup- 

 ported in position normal to the axis of the microscope 

 by means of a stand with an adjustable arm placed to 

 the left of the microscope. It is recommended to have 

 a series of cells, ready for filling with various media of 

 the most frequently required refractive power. The 

 advantages of Adams' method of determining optic axial 

 angles may also be combined with those of the method 

 now described, by use of a cell consisting of an upper 

 cylindrical portion terminating below in a sphere filled 

 with the liquid. As regards suitable liquids, an admir- 

 able list is given by Herr Pulfrich in his book descrip- 

 tive of the construction and use of the total reflectome- 

 ter recently devised by him (p. 64). Two errors in that 

 list, however, are corrected by Prof. Klein; he has been 

 unable to prepare the solution of mercuric iodide in ani- 



